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Description
(In my files, fun fact, it's titled 'sans.' like his theme song.)
Sans's Soul: starryoak.deviantart.com/art/S…
Design Notes:
Doing this before the headcanon dump, shockingly! Cause the headcanon dump is pretty much the bulk of this post, I figure you deserve to have the design out of the way; I gave him his eyelights from the flashing eye sequence, the color is simply constant for him, and I draw him with bags under his eyesockets (he's a magical skeleton, don't ask), as his sprite has unexplained lines under his eyes that give the impression of bags under them. His smile looks intentionally forced. He's smiling because he has to. He wears pink slippers most of the time (occasionally mixes it up with his blue ones) and wears socks under his slippers, because why the fuck not. Plus, we know that Sans wears socks, considering the hilarious conversation in post-it notes that he and Papyrus had. So yeah, socks and slippers. He wears a turtleneck under his hoodie.
You know I would have to draw him eventually. I waited until I'd finished Papyrus to post this, but man, it's kind of expected that I'd draw him. As the most popular character in Undertale and all. And... Yeah, I do love him. If only because of the fact that I'm a sucker for existential horror. And you know what? That's exactly what Sans represents. His story and backstory is exactly the most horrifying thing I can think of... So horrifying that I actually wrote about it before I ever knew Undertale existed. You might remember it as the writing that got me a Daily Deviation (I still can't believe that happened). It occurred to me while I did my usual obsessive headcanoning that Sans's situation closely resembles the one depicted in 'It's The End of The World Today' , except with a slightly less bleak outlook. I only say slightly. Because I'm... I'm not a nice person to characters I like. I really, really am not. I love hurting them.
I'm a sadist. I think all of my Total Drama fans know that the characters I love most are the ones who have been through hell, as well as anyone who pays enough attention, I tend to always overthink every little thing ever put down on canon and taking it to heart.
I also... I think about a lot of the topics that Sans's situation brings up, because of my personal fears and fascinations. A bit more than is healthy. So even before I really knew about Undertale, I'd already thought about the concept of Groundhog Day's Loops, and how, in a realistic setting, they would affect a person mentally. In fact, considering how fucked up resets are, my interpretation is actually a bit lax on how fucked up he would be. If I was to take it to it's most realistic, objective level, Sans would be an irreparable mess. But because I want to be optimistic, I've actually made him less fucked up than he should be by the resets. If I was being less optimistic, Sans would have a hard time treating other people as people after seeing them as puppets for so long. He would have no empathy because it doesn't matter. But I'm, at the end, an optimist. I may put him through hell, but I do want him to get better.
So, anyway, let's get down to me rambling for hours about all the ways that Sans is a fucked up wreck.
As I've previously established with my W.D. Gaster headcanons, for one; Gaster is Sans and Papyrus's father in them. But, unlike a lot of people like to consider, he's not evil. Not ragging on the morally ambiguous or detestable Gaster HC's, but I prefer my Goopster as a Goodster (this is basically a balm for any pain I'm gonna cause you later). Generally, it fell on Sans to be the responsible adult of the house(even at 13, when Gaster blasted holes through his hands), handling the various things like 'putting the fridge in the groceries' when Gaster got home from work, keeping track of the 4 year old hyperactive Papyrus, generally being the one doing things. This does not mean he didn't screw around with his dad on occasion, especially with the one time he did in fact ask him to 'put the fridge in the groceries', which ended with Sans figuring out how to make the grocery bag stretch over the fridge. Don't ask for what you don't want, old man~.
Sans eventually joined his dad in on the science experiments, being extremely fascinated in the CORE that was being built to provide energy to everyone in the Underground, basically becoming his intern, though their relationship was much less formal.
Sans was there, and present.. in the most extreme way, for Gaster's death. When the CORE began to self implode, Gaster and his assistants each decided to work on the CORE on different levels, with Gaster tackling the base, knowing that it would be the most important piece, and one only he knew how to operate, taking his son with him for back up.
What followed was basically a scene not unlike the climax of The Wrath Of Khan. Guess who was Spock and who was Kirk in this situation? The CORE was slowly malfunctioning and would take out the entirety of the Underground if not properly maintained... from within. The exact place where the CORE was destabilizing.
As he realized he would die if he went in, accepting this, Gaster quickly applied the remaining extracted DETERMINATION into himself, hoping against hope that his SOUL would last and help to advance the field of science, and allow his sons and all monsterkind to break down the barrier and live free on the surface... or just a desperate bid to live long enough to work the controls to keep it from fully breaking down, even if he broke down.
This all occurred with Sans pressed up against the other side of the CORE controls... Yeah, it can't have been very fun at all for Sans to watch his father melting away, scattering across time and space. Gaster's atoms slowly ripped apart as his body melted completely, his body collapsing in on itself in erratic amounts, but eventually evaporating entirely, all while assuring Sans that 'everything will be fine' as Sans attempted to blast the door open, not wanting to lose him. But, well, you know what happened.
As Dr. Gaster melted away at the spatiotemporal level, everyone's memories of him ever having existed began to ebb away. His own son Papyrus forgot that he ever even had a father, only remembering that he had a brother Sans.
As for Sans... Sans remembered, perhaps because of his proximity to the event, perhaps because the event was burned into his mind, perhaps because the event did manage to affect him slightly before Gaster sealed himself off, that being the reason he rushed into the CORE, seeing his son could be affected by the deteriorating CORE... the exact events are unclear and slightly unimportant.
What is important is that, with his perplexed little brother who was barely out of being a first grader at 8, and himself only 17, Sans left the CORE and Hotland behind, taking Papyrus to the snowy region of... Snowdin (Thanks, Asgore). The pair asserted themselves upon the doldrums of the denizens, bringing a great deal more excitement to the environment.
The events between then and when the resets started affecting Sans is a story for another time. But it's still kind of a trainwreck of nightmares and neurosis and alcoholism (he drinks ketchup now, but again, that's a story for another time).
Sans, between the resets Flowey did and the resets Frisk/Chara went through, has lived.... a long time. But he couldn't tell you how long; counting the numbers becomes impossible with enough time blurring it together, with Flowey's resets not being as short, it could be months of time between the start of a time and the time where it would reset. But we know, in the end, that Flowey had to have earned enough LOVE to reach LVL 9999 (according to his save file), and considering the almost exponential increases involved in the EXP needed for each level, with level 20 alone requiring 50,000 EXP.... Flowey would have to have killed the Underground approximately 500 times to accumulate that much EXP.... And as Flowey brought up; Sans was there for every single one of them.
Flowey, unlike Chara, was not solely motivated by a lust for murder and LV, he toyed with the residents of the Underground rather than carve a bloody swath through it. He, as I mentioned above, is the BAD END for a person trapped in resets; something or someone who stops believing in empathy or the free will of anyone that they murder. And, with Sans Karmic Retribution powers, the irony is, the more powerful that Flowey was, the more LOVE he had in him, the easier it was for Sans to strike down, even if he still only has 1 HP.
.. Did you know, in my headcanon, HP stands for 'HoPe'? Just a fact. 1 HP is minimum HoPe a monster can have as a base HP; any less, and they die, literally out of a lack of will to live. It can be depleted physically, but the only way someone's HP decreases in base stats is to lose hope. Sans was born with 1 HP as a base, but it can be raised, without LOVE, at least for Monsters, by gaining hope. When Gaster was alive, Sans's HP slowly rose to 5, before Gaster's death knocked it down to 1 again. Papyrus growing up made it easier, but the hope was once again lost when Flowey began resetting. He'll never really reach HP levels above double digits, he could never reach above 15... and mentally, he's in a place where it's almost guaranteed he'll be at 1 HP forever.
Sans took to the resets about as well as you expect; when they first occurred, he freaked the hell out; waking up one day and realizing it's months ago... it's not easy. But at least Flowey's first resets were good ones. But the extended amount of time, it gets to you... What's the point? The only thing he really lived for was to keep Papyrus happy, so he clung onto life, even as Flowey killed them all over and over again. Sure, sometimes he let it slip out, but Papyrus noticing would always end eventually.. whether it was because reality was reset or because Flowey murdered him. He learned how to hide it, though, and learned how to grin and bear it.
Which is a fun segue to something I want to point out; Sans... is almost the perfect Stepford Smiler. We know that Sans is capable of stopping his smile, but we only learn that when he's on his deathbed; he'll only slightly stop smiling , and still try to pull another grin when he starts bleeding, because that's how deep it seems to be ingrained in him to keep smiling. He can physically stop... but he doesn't let himself. Nothing, killing you, intimidating Frisk, everything he does, he does with a smile. Not because he's happy, but because he can't stop smiling. He won't. Because it allows him to hide his emotions under a mask, because it was the easiest way to fool people when the rresets started. Just keep smiling. It's like how he's always joking. Eventually, after so much time spent resetting, hiding his emotions behind that smile has become so deeply ingrained in him that the only time he lets himself stop smiling even a little is when you've stabbed him to death.
So basically, if I ever draw Sans, no matter what his feelings are... he'll always be smiling. Because that's how he has basically coasted by. Pretend to be happy and don't let anyone see.
It's one of the many ways the resets fucked him up so badly.
He can't stop grinning, you can't stop smiling, you can't stop joking, if you do, they'll know, you just keep smiling. Even after they know, it's hard to express any emotion for Sans. Or at least, express it physically.
But seriously, I did contemplate the fact that Sans is actively smiling throughout the whole game, since we know he can physically not smile, but no matter what occurs in game, he never stops. So he's forcing himself to smile constantly... And if you force it long enough, well, your face doesn't actually get stuck that way, but mentally, it does. Plus, it makes him feel safer, knowing they can't see what he's feeling inside. It's a wall between his emotions being seen by others. Nobody needs to know he's upset, why bother? He's not worth worrying about anyway.
Even after they find out that he's forcing himself to smile, it's hard to break that habit of hiding all your emotions, even if they say it's ok not to smile, that they want him to express his emotions physically, it's been so long since he hasn't been hiding those emotions... He'd break down eventually, but that mentality's still there. Still gotta keep smiling.
Some more info because I like headcanon dumping, Sans can actually tell the time surprisingly well. don't mean, you know, hours and minutes, but the days. I say surprisingly because it's no simple feat to keep an internal clock going when the same three days kept repeating. But since the timeline stayed the same, consistent, it was... not as hard as it could have been. But he does have trouble with any date that occurred before the current set of resets began... it's been too long for him to remember time before that clearly enough. He counts forward from the day Frisk fell down, because it's the only fixed time point he can remember.
Frisk fell on a Tuesday. Frisk left the ruins on Wednesday. Frisk freed the monsters at the end of Wednesday. The first day outside was Thursday.
Count forward. His default answer to what day it is is 'wednesday' because it was Wednesday for so long for him.
But he does know sadly pretty clockwork the schedule as long as he starts from September 15... just give him time to work out the rest. He literally can count forward for as long as it takes, because he.... still counts how many days it's been since the last reset. Don't tell Frisk, he doesn't want to upset them. The calendar they gave him is really helpful when they get him that but he still reflexively tries to count like that for as long as it takes
At first after they found out all about what happened they just assumed he couldn't tell dates that well, it took him so long to get the right answer.
But somebody tried to ask him if he was having trouble and accidentally set him off counter.
Alphys: Uh... S-Sans, do you... n-need us to tell y-you?
Sans: then it was fri-damnit!
Alphys: I-I'm... s-sorry?
Sans: you set me off! now i have to start over!
Alphys: W-what?
Sans: Frisk fell on tuesday, that was ___ days ago, then wednesday, the barrier fell, the first day out was thursday, next it was friday, then sunday, then-"
Alphys: Sans.. a-are you c-counting t-the days l-linearly?"
Sans: goddamnit! again! what did i just say? tuesday, wednesday, thursday, friday, sunday-
Alphys: I think w-we have t-to get you a c-calendar.... I'm g-going to call P-Papyrus.
Sans: [several minutes later] it's saturday. got it."
Alphys: W-well, y-you're r-right...
he has it perfectly well figured out just give him like five minutes and stop interrupting him
To give him credit, it's pretty impressive that he can count forwards from a date and not lose track of his count and reach the correct date, but we have calendars for that... and that's how they got the calendar. It's practically the only possible interesting story behind a calendar anyone's ever had. But if you don't tell him to think about it, he will default to saying 'wednesday'. Because that's when all the important things happened in his life. For him, the world started on Tuesday. So he basically goes 'it's wednesday, right?' but if you ask him to actually think, you're going to have to wait like five minutes and not interrupt him, but he'll get the right answer eventually.
Sans also, as a result of the resets, has issues with object and action permanence, often forgetting that his life has consequences now, since before, he was so utterly helpless that he could effectively do anything without it ever actually mattering in the grand scheme of things... but once he figures out the problems of action permanence, that creates another problem for him.
Another result is that he has trouble with conversations in general. He says the same things every time you play, with minor variations, always. He knows what to say. He's done a lot of different things during the resets, so many that he knows exactly what happens whenever he does something. He knows how everyone will react to everything he does. He knows every line that they say. He knows what everyone is about to say before they say it. He knows exactly what will happen if he says something, he has every conversation he could possibly have memorized. ..... Or at least, he does underground
He's never been above ground for as long as he has in the final reset. He's afraid of conversing with people he doesn't know at all, because he has no idea how they'd react to anything he'll say, he doesn't know what to say, and that's such a new idea to him; conversations where he doesn't know everything that's going to be said and what will happen are terrifying.
He has a vague idea of some of what his friends will react in general, but he can't predict what they'll do perfectly.
Imagine if you knew everything everyone could possibly say to you and everything you can say to them and how they'd react... You'd know your lines perfectly. You know the script well. You have a safe barrier of knowing what is the perfect thing to say in every situation. You can take comfort in knowing exactly how everything will go. Now imagine if you suddenly lost that ability.
Every conversation is not only permanent, but you have no idea what they'll say. You have no idea what they'll do when you say something.
Not just permanent, you have no idea what the perfect answer is anymore
Imagine how scary that would be?
And imagine how hard decision making is now that you don't know what the end result would be.
Imagine being paralyzed with indecision over even basic decisions because everything is completely permanent
You have to make the right decision, because there's no going back
That's how Sans has to live; with the sudden realization that whatever he does or says is permanent.
He's working on accepting and understanding that he doesn't have to say the exact perfect thing.
And that conversations with people are supposed to be natural. It's natural not to know what another person will say.
But for a while, he'll just be stuck staring at decisions, even minor ones; if he cleans up his trash tornado, he can never get it back and even making it again, it will never be the same again.
So Papyrus will find him just staring at that tornado, debating cleaning it for about an hour, because he just doesn't know if he should clean it up because he knows this is an irreversible decision. No matter how you point out that the decision is a minor, inconsequential, unneeded one, it's still permanent.
And that terrifies him.
so uh yeah... Sans is a very fun character for me. I have all this backstory and canon story to work with that basically paints a miserable picture that I can dig into and make into the best ever things. I didn't even cover all of my headcanons, man, this is like only half of them, maybe.

Description
(In my files, fun fact, it's titled 'sans.' like his theme song.)
Sans's Soul: starryoak.deviantart.com/art/S…
Design Notes:
Doing this before the headcanon dump, shockingly! Cause the headcanon dump is pretty much the bulk of this post, I figure you deserve to have the design out of the way; I gave him his eyelights from the flashing eye sequence, the color is simply constant for him, and I draw him with bags under his eyesockets (he's a magical skeleton, don't ask), as his sprite has unexplained lines under his eyes that give the impression of bags under them. His smile looks intentionally forced. He's smiling because he has to. He wears pink slippers most of the time (occasionally mixes it up with his blue ones) and wears socks under his slippers, because why the fuck not. Plus, we know that Sans wears socks, considering the hilarious conversation in post-it notes that he and Papyrus had. So yeah, socks and slippers. He wears a turtleneck under his hoodie.
You know I would have to draw him eventually. I waited until I'd finished Papyrus to post this, but man, it's kind of expected that I'd draw him. As the most popular character in Undertale and all. And... Yeah, I do love him. If only because of the fact that I'm a sucker for existential horror. And you know what? That's exactly what Sans represents. His story and backstory is exactly the most horrifying thing I can think of... So horrifying that I actually wrote about it before I ever knew Undertale existed. You might remember it as the writing that got me a Daily Deviation (I still can't believe that happened). It occurred to me while I did my usual obsessive headcanoning that Sans's situation closely resembles the one depicted in 'It's The End of The World Today' , except with a slightly less bleak outlook. I only say slightly. Because I'm... I'm not a nice person to characters I like. I really, really am not. I love hurting them.
I'm a sadist. I think all of my Total Drama fans know that the characters I love most are the ones who have been through hell, as well as anyone who pays enough attention, I tend to always overthink every little thing ever put down on canon and taking it to heart.
I also... I think about a lot of the topics that Sans's situation brings up, because of my personal fears and fascinations. A bit more than is healthy. So even before I really knew about Undertale, I'd already thought about the concept of Groundhog Day's Loops, and how, in a realistic setting, they would affect a person mentally. In fact, considering how fucked up resets are, my interpretation is actually a bit lax on how fucked up he would be. If I was to take it to it's most realistic, objective level, Sans would be an irreparable mess. But because I want to be optimistic, I've actually made him less fucked up than he should be by the resets. If I was being less optimistic, Sans would have a hard time treating other people as people after seeing them as puppets for so long. He would have no empathy because it doesn't matter. But I'm, at the end, an optimist. I may put him through hell, but I do want him to get better.
So, anyway, let's get down to me rambling for hours about all the ways that Sans is a fucked up wreck.
As I've previously established with my W.D. Gaster headcanons, for one; Gaster is Sans and Papyrus's father in them. But, unlike a lot of people like to consider, he's not evil. Not ragging on the morally ambiguous or detestable Gaster HC's, but I prefer my Goopster as a Goodster (this is basically a balm for any pain I'm gonna cause you later). Generally, it fell on Sans to be the responsible adult of the house(even at 13, when Gaster blasted holes through his hands), handling the various things like 'putting the fridge in the groceries' when Gaster got home from work, keeping track of the 4 year old hyperactive Papyrus, generally being the one doing things. This does not mean he didn't screw around with his dad on occasion, especially with the one time he did in fact ask him to 'put the fridge in the groceries', which ended with Sans figuring out how to make the grocery bag stretch over the fridge. Don't ask for what you don't want, old man~.
Sans eventually joined his dad in on the science experiments, being extremely fascinated in the CORE that was being built to provide energy to everyone in the Underground, basically becoming his intern, though their relationship was much less formal.
Sans was there, and present.. in the most extreme way, for Gaster's death. When the CORE began to self implode, Gaster and his assistants each decided to work on the CORE on different levels, with Gaster tackling the base, knowing that it would be the most important piece, and one only he knew how to operate, taking his son with him for back up.
What followed was basically a scene not unlike the climax of The Wrath Of Khan. Guess who was Spock and who was Kirk in this situation? The CORE was slowly malfunctioning and would take out the entirety of the Underground if not properly maintained... from within. The exact place where the CORE was destabilizing.
As he realized he would die if he went in, accepting this, Gaster quickly applied the remaining extracted DETERMINATION into himself, hoping against hope that his SOUL would last and help to advance the field of science, and allow his sons and all monsterkind to break down the barrier and live free on the surface... or just a desperate bid to live long enough to work the controls to keep it from fully breaking down, even if he broke down.
This all occurred with Sans pressed up against the other side of the CORE controls... Yeah, it can't have been very fun at all for Sans to watch his father melting away, scattering across time and space. Gaster's atoms slowly ripped apart as his body melted completely, his body collapsing in on itself in erratic amounts, but eventually evaporating entirely, all while assuring Sans that 'everything will be fine' as Sans attempted to blast the door open, not wanting to lose him. But, well, you know what happened.
As Dr. Gaster melted away at the spatiotemporal level, everyone's memories of him ever having existed began to ebb away. His own son Papyrus forgot that he ever even had a father, only remembering that he had a brother Sans.
As for Sans... Sans remembered, perhaps because of his proximity to the event, perhaps because the event was burned into his mind, perhaps because the event did manage to affect him slightly before Gaster sealed himself off, that being the reason he rushed into the CORE, seeing his son could be affected by the deteriorating CORE... the exact events are unclear and slightly unimportant.
What is important is that, with his perplexed little brother who was barely out of being a first grader at 8, and himself only 17, Sans left the CORE and Hotland behind, taking Papyrus to the snowy region of... Snowdin (Thanks, Asgore). The pair asserted themselves upon the doldrums of the denizens, bringing a great deal more excitement to the environment.
The events between then and when the resets started affecting Sans is a story for another time. But it's still kind of a trainwreck of nightmares and neurosis and alcoholism (he drinks ketchup now, but again, that's a story for another time).
Sans, between the resets Flowey did and the resets Frisk/Chara went through, has lived.... a long time. But he couldn't tell you how long; counting the numbers becomes impossible with enough time blurring it together, with Flowey's resets not being as short, it could be months of time between the start of a time and the time where it would reset. But we know, in the end, that Flowey had to have earned enough LOVE to reach LVL 9999 (according to his save file), and considering the almost exponential increases involved in the EXP needed for each level, with level 20 alone requiring 50,000 EXP.... Flowey would have to have killed the Underground approximately 500 times to accumulate that much EXP.... And as Flowey brought up; Sans was there for every single one of them.
Flowey, unlike Chara, was not solely motivated by a lust for murder and LV, he toyed with the residents of the Underground rather than carve a bloody swath through it. He, as I mentioned above, is the BAD END for a person trapped in resets; something or someone who stops believing in empathy or the free will of anyone that they murder. And, with Sans Karmic Retribution powers, the irony is, the more powerful that Flowey was, the more LOVE he had in him, the easier it was for Sans to strike down, even if he still only has 1 HP.
.. Did you know, in my headcanon, HP stands for 'HoPe'? Just a fact. 1 HP is minimum HoPe a monster can have as a base HP; any less, and they die, literally out of a lack of will to live. It can be depleted physically, but the only way someone's HP decreases in base stats is to lose hope. Sans was born with 1 HP as a base, but it can be raised, without LOVE, at least for Monsters, by gaining hope. When Gaster was alive, Sans's HP slowly rose to 5, before Gaster's death knocked it down to 1 again. Papyrus growing up made it easier, but the hope was once again lost when Flowey began resetting. He'll never really reach HP levels above double digits, he could never reach above 15... and mentally, he's in a place where it's almost guaranteed he'll be at 1 HP forever.
Sans took to the resets about as well as you expect; when they first occurred, he freaked the hell out; waking up one day and realizing it's months ago... it's not easy. But at least Flowey's first resets were good ones. But the extended amount of time, it gets to you... What's the point? The only thing he really lived for was to keep Papyrus happy, so he clung onto life, even as Flowey killed them all over and over again. Sure, sometimes he let it slip out, but Papyrus noticing would always end eventually.. whether it was because reality was reset or because Flowey murdered him. He learned how to hide it, though, and learned how to grin and bear it.
Which is a fun segue to something I want to point out; Sans... is almost the perfect Stepford Smiler. We know that Sans is capable of stopping his smile, but we only learn that when he's on his deathbed; he'll only slightly stop smiling , and still try to pull another grin when he starts bleeding, because that's how deep it seems to be ingrained in him to keep smiling. He can physically stop... but he doesn't let himself. Nothing, killing you, intimidating Frisk, everything he does, he does with a smile. Not because he's happy, but because he can't stop smiling. He won't. Because it allows him to hide his emotions under a mask, because it was the easiest way to fool people when the rresets started. Just keep smiling. It's like how he's always joking. Eventually, after so much time spent resetting, hiding his emotions behind that smile has become so deeply ingrained in him that the only time he lets himself stop smiling even a little is when you've stabbed him to death.
So basically, if I ever draw Sans, no matter what his feelings are... he'll always be smiling. Because that's how he has basically coasted by. Pretend to be happy and don't let anyone see.
It's one of the many ways the resets fucked him up so badly.
He can't stop grinning, you can't stop smiling, you can't stop joking, if you do, they'll know, you just keep smiling. Even after they know, it's hard to express any emotion for Sans. Or at least, express it physically.
But seriously, I did contemplate the fact that Sans is actively smiling throughout the whole game, since we know he can physically not smile, but no matter what occurs in game, he never stops. So he's forcing himself to smile constantly... And if you force it long enough, well, your face doesn't actually get stuck that way, but mentally, it does. Plus, it makes him feel safer, knowing they can't see what he's feeling inside. It's a wall between his emotions being seen by others. Nobody needs to know he's upset, why bother? He's not worth worrying about anyway.
Even after they find out that he's forcing himself to smile, it's hard to break that habit of hiding all your emotions, even if they say it's ok not to smile, that they want him to express his emotions physically, it's been so long since he hasn't been hiding those emotions... He'd break down eventually, but that mentality's still there. Still gotta keep smiling.
Some more info because I like headcanon dumping, Sans can actually tell the time surprisingly well. don't mean, you know, hours and minutes, but the days. I say surprisingly because it's no simple feat to keep an internal clock going when the same three days kept repeating. But since the timeline stayed the same, consistent, it was... not as hard as it could have been. But he does have trouble with any date that occurred before the current set of resets began... it's been too long for him to remember time before that clearly enough. He counts forward from the day Frisk fell down, because it's the only fixed time point he can remember.
Frisk fell on a Tuesday. Frisk left the ruins on Wednesday. Frisk freed the monsters at the end of Wednesday. The first day outside was Thursday.
Count forward. His default answer to what day it is is 'wednesday' because it was Wednesday for so long for him.
But he does know sadly pretty clockwork the schedule as long as he starts from September 15... just give him time to work out the rest. He literally can count forward for as long as it takes, because he.... still counts how many days it's been since the last reset. Don't tell Frisk, he doesn't want to upset them. The calendar they gave him is really helpful when they get him that but he still reflexively tries to count like that for as long as it takes
At first after they found out all about what happened they just assumed he couldn't tell dates that well, it took him so long to get the right answer.
But somebody tried to ask him if he was having trouble and accidentally set him off counter.
Alphys: Uh... S-Sans, do you... n-need us to tell y-you?
Sans: then it was fri-damnit!
Alphys: I-I'm... s-sorry?
Sans: you set me off! now i have to start over!
Alphys: W-what?
Sans: Frisk fell on tuesday, that was ___ days ago, then wednesday, the barrier fell, the first day out was thursday, next it was friday, then sunday, then-"
Alphys: Sans.. a-are you c-counting t-the days l-linearly?"
Sans: goddamnit! again! what did i just say? tuesday, wednesday, thursday, friday, sunday-
Alphys: I think w-we have t-to get you a c-calendar.... I'm g-going to call P-Papyrus.
Sans: [several minutes later] it's saturday. got it."
Alphys: W-well, y-you're r-right...
he has it perfectly well figured out just give him like five minutes and stop interrupting him
To give him credit, it's pretty impressive that he can count forwards from a date and not lose track of his count and reach the correct date, but we have calendars for that... and that's how they got the calendar. It's practically the only possible interesting story behind a calendar anyone's ever had. But if you don't tell him to think about it, he will default to saying 'wednesday'. Because that's when all the important things happened in his life. For him, the world started on Tuesday. So he basically goes 'it's wednesday, right?' but if you ask him to actually think, you're going to have to wait like five minutes and not interrupt him, but he'll get the right answer eventually.
Sans also, as a result of the resets, has issues with object and action permanence, often forgetting that his life has consequences now, since before, he was so utterly helpless that he could effectively do anything without it ever actually mattering in the grand scheme of things... but once he figures out the problems of action permanence, that creates another problem for him.
Another result is that he has trouble with conversations in general. He says the same things every time you play, with minor variations, always. He knows what to say. He's done a lot of different things during the resets, so many that he knows exactly what happens whenever he does something. He knows how everyone will react to everything he does. He knows every line that they say. He knows what everyone is about to say before they say it. He knows exactly what will happen if he says something, he has every conversation he could possibly have memorized. ..... Or at least, he does underground
He's never been above ground for as long as he has in the final reset. He's afraid of conversing with people he doesn't know at all, because he has no idea how they'd react to anything he'll say, he doesn't know what to say, and that's such a new idea to him; conversations where he doesn't know everything that's going to be said and what will happen are terrifying.
He has a vague idea of some of what his friends will react in general, but he can't predict what they'll do perfectly.
Imagine if you knew everything everyone could possibly say to you and everything you can say to them and how they'd react... You'd know your lines perfectly. You know the script well. You have a safe barrier of knowing what is the perfect thing to say in every situation. You can take comfort in knowing exactly how everything will go. Now imagine if you suddenly lost that ability.
Every conversation is not only permanent, but you have no idea what they'll say. You have no idea what they'll do when you say something.
Not just permanent, you have no idea what the perfect answer is anymore
Imagine how scary that would be?
And imagine how hard decision making is now that you don't know what the end result would be.
Imagine being paralyzed with indecision over even basic decisions because everything is completely permanent
You have to make the right decision, because there's no going back
That's how Sans has to live; with the sudden realization that whatever he does or says is permanent.
He's working on accepting and understanding that he doesn't have to say the exact perfect thing.
And that conversations with people are supposed to be natural. It's natural not to know what another person will say.
But for a while, he'll just be stuck staring at decisions, even minor ones; if he cleans up his trash tornado, he can never get it back and even making it again, it will never be the same again.
So Papyrus will find him just staring at that tornado, debating cleaning it for about an hour, because he just doesn't know if he should clean it up because he knows this is an irreversible decision. No matter how you point out that the decision is a minor, inconsequential, unneeded one, it's still permanent.
And that terrifies him.
so uh yeah... Sans is a very fun character for me. I have all this backstory and canon story to work with that basically paints a miserable picture that I can dig into and make into the best ever things. I didn't even cover all of my headcanons, man, this is like only half of them, maybe.