Birthday Math
CALENDARS
An “Equestria Girls” fanfic by Erin Mills
“My Little Pony: Equestria Girls” ©2019 Hasbro/DHX Media
The scream could be heard at least three blocks away. In Pinkie Pie's bedroom, Fluttershy cautiously removed her hands from her ears.
“Um, Pinkie? Are you okay?” she asked.
Pinkie Pie was splayed out face up on the floor, hyperventilating. Her eyes stared blankly at the ceiling. A calendar was lying next to her, tented from where it had landed when Pinkie realized her mistake.
“How?” the pink party girl whispered. “How did I forget to put Sunset's birthday on my calendar?”
“Um... I'm sure you didn't mean to,” Fluttershy ventured. She frowned as a thought crossed her mind. “Actually, I don't think that I know when her birthday is, now that I think about it.”
“WHAT?!” Pinkie went from lying down to sitting upright with no discernible movement in between. “How can you not know her birthday either?”
“To be honest, I never thought to ask.” She pursed her lips as she thought. “Pinkie, have we ever done anything for Sunset's birthday?”
Pinkie's brow furrowed. She got up and went to her desk. She yanked open a drawer (which pulled out much further than it logically should have been able to) and began riffling through the files lined up neatly inside.
“September... October... November...” she muttered as she flipped the pages in each folder. She looked up and slammed the drawer shut. She whirled around to face her friend. Fluttershy let out an “eep” at the wild stare that had formed on Pinkie's face. Another “eep” followed when Pinkie grabbed her by the shoulders.
“We. Have. NEVER. Done. Anything. For. Sunset's. BIRTHDAY!” Pinkie's eyes bored into Fluttershy's then filled with tears and the party girl collapsed on the floor, sobbing wildy and drenching Fluttershy's shoes. “I'm the worst friend eh-eh-eh-EVERRRRR!”
Fluttershy tried not to wince at the squelching noise her shoes made as she leaned down to pat Pinkie on the shoulder. “I'm sure it's okay, Pinkie. I mean, I didn't know when her birthday was either. I don't think she's ever told us!”
In an instant, Pinkie was back on her feet and posing dramatically. “NO! I will not stand idly by and let one of our best friends' birthdays go uncelebrated! It's wrong! It's mean! It's downright unpatriotic!”
Flutteshy frowned. “I don't think that's what you mean.”
“Well, I'm gonna do something about it!” Pinkie said, ignoring her. “I'm gonna find out Sunset's birthday and then I'm gonna throw her the bestest brithday party ever! With cake! And ice cream! And enough streamers and balloons to choke a horse!”
“Oh, don't do that. Poor horsey.”
But Pinkie was on a roll. “I'll even get a clown! No! Two clowns! No! A whole clown car full of clowns clowning it up all over the place! And a mime for good measure!”
“But Sunset's terrified of mimes...”
“And to top it all off,” Pinkie continued unheeded. “Strippers!”
Fluttershy blinked. “S... strippers?”
“A whole company of them! Big muscular strippers with the tiniest of thongs!”
“Oh... my. Wait... doesn't Sunset like girls too?”
“A co-ed company of strippers with the tiniest of thongs! Boy strippers! Girl strippers! Non-binary strippers who are just as valid as any other stripper or person in the sex work industry! All with the biggest bazooms that correspond to their preferred gender identity! All slathered in oil!”
“... oil?” Fluttershy squeaked.
“SLATHERED!” Pinkie confirmed. “A whole company of gender inclusive, giant bazoomed strippers in tiny, tiny thongs slathered in oil!”
“... maybe chocolate syrup?”
“YES!” Pinkie cried. “Even better! And whipped cream!”
“... mmmm...”
“BUT!” Pinkie whirled around, causing Fluttershy to quickly remover her hand from under her skirt. “Before that, we need to find out Sunset's birthday!”
Fluttershy blinked. “Right,” she said. “So what do we do? Get dressed up in stealthy outfits, break into her apartment, find anything that might give us a clue as to her birthday and sneak out again?”
Now it was Pinkie's turn to blink. “Wow, Fluttershy. Breaking into a friend's house? That's the first place you go?”
Fluttershy blushed. “Um... sorry. I guess I got carried away.”
Pinkie flopped onto the floor again and hugged her friend. “That's okay! I won't tell anyone about your evil criminal tendencies.”
“Um... thank you?”
“You're welcome, silly!”
“So what do we do then?”
“Well, DUH!”
“I... don't know if I can answer that question,” Sunset said. She, Pinkie, and Fluttershy were in their usual spot at Sugarcube Corner. Coffee, tea, and a sugar infused caffiene abomination unto God and Gordon Ramsay were in evidence.
“What do you mean?” Fluttershy asked.
“Yeah,” Pinkie chimed. “How can you not answer 'Hey, Sunset, when's your birthday?'”
Sunset blushed and rubbed the back of her head. “Well... because... I'm not really sure when it is.”
“What?!” yelped Pinkie.
Fluttershy looked confused. “I don't understand. You had to put your birthday on your driver's license, right?”
“That's not actually my birthday,” Sunset explained. “It's just the date on all my paperwork. I picked one at random so I had something to put in the box.”
“WHAT?!” Pinkie exclaimed.
Fluttershy's confused expression continued. “I'm still not following you. You have a birthday in Equestria, right?”
Sunset nodded. “But the problem is that Equestria doesn't use the same calendar we do over here.”
“WHA-MMMPH!” Pinkie's latest squeal was cut off when Sunset stuffed a cruller into the girl's open mouth.
“So, you're saying there's some kind of time difference between here and there?” Fluttershy asked. “Like a week here is a month there or something?”
Sunset shook her head. “No. Time flows at the same rate in both worlds, the difference is in how that time is measured.”
Pinkie swallowed the cruller whole with a loud gulp then looked at Sunset. “I don't get it.”
Sunset sighed. “Okay, it's like this. Most units of time measurement over here are based on natural phenomena. Humans worked out that the planet completes one rotation in 24 hours. That became the standard day. The moon orbits the planet once every 30 or so days and that became the basis for a month. Then you worked out that it takes roughly 365 days for the planet to orbit the sun and that became the basis for a year.
“But in Equestria, the sun and the moon are controlled by Princess Celestia and Princess Luna. Which means that day and night can be as long as they want. That's part of why Princess Luna became so resentful and became Nightmare Moon, but that's a whole other story. Anyway, back when they first became the rulers of Equestria, time measurement was a mess. Each tribe of ponies had their own calendar and it made trade and communication between the tribes difficult. If a pegasus received a letter from an earth pony to trade on the 26th, which calendar you use? The earth pony could show up a full week before the pegasus thinks they're supposed to meet. The princesses established what's now known as the Equestria Standard Calendar.”
Sunset took a sip of her tea and continued. “By one of those weird coincidences, they established that there were sixty seconds in a minute and sixty minutes in an hour, just like over here, but the difference comes in the larger units. To make things easy on everypony – “
Fluttershy giggled. “'Everypony.' Still so cute.”
Sunset rolled her eyes, but she was smiling as she did. “Anyway, to make things easy, the princesses established that there would be 20 hours in a day: ten of day and ten of night on the dates decided to be the equinoxes. They adjusted the length of day and night throughout the year, based on crop cycles and weather needs. But an Equestrian day would be 20 hours long in total. They decided to move exponentially and decreed an Equestrian year would be 200 days long.”
“Wait, they can just do that?” Pinkie asked.
“Well, yeah,” Sunset said. “They move the sun and moon, so they can determine how long a day is and by extension, how long a year is. There's even evidence that similar things happened over here. Two Roaman emperors wanted to be represented in the calendar so they shoved two extra months into it. That's why October isn't the eighth month of the year, but the tenth, despite the name.”
“But... does that mean Equestria is flat? Where do the sun and moon go when they're not in the sky? I NEED ANSWERS, SUNSET!”
Sunset yelped as Pinkie slammed her hands down and thrust herself into Sunset's face. Sunset put her tea down and gently pushed Pinkie back into her seat. “I don't really feel like going into all that right now, Pinkie, okay? It's really complicated and I think it might actually be a state secret, so I'd rather not open that can of worms.”
“Okay!” Pinkie smiled and took another sip of her liquid cavity.
“I still don't see how that's not a time difference,” Fluttershy said. “The years aren't the same length, so doesn't that mean time passes more rapidly over there than over here?”
Sunset shook her head. “Years and days are different lengths on other planets in this solar system, aren't they? I mean, on one of them it takes longer for the planet to rotate than to orbit the sun, so its day is longer than its year. But if I tell you that it takes five years on this planet for another planet to orbit the sun, the same amount of time has passed for both planets, it's just measured differently since it's been established over here that one year equals the amount of time it takes a planet to orbit the sun. The same thing is true between here and Equestria.”
“Okay, I think I understand,” Fluttershy said. Her eyes widened. “Wait, doesn't that mean, with shorter Equestrian years, that you're actually older over there?”
Sunset nodded. “Yep.”
Fluttershy thought for a moment. “You must have been an Equestrian adult when you came over! How old are you really, Sunset? In Equestrian years.”
Sunset sighed. “I think I'm about 39, but I'm not entirely sure. When I came over, I was 26.”
“WHAT?!” Pinkie's screech caused everyone to stare at them. Sunset and Fluttershy blushed and glared at her. Pinkie soon had a blush of her own and sat back down.
“Sorry,” she said sheepishly. Fluttershy looked back at Sunset.
“So, how old were you in human years?”
“When I came through, I had the body of someone who was roughly twelve years old,” Sunset answered. She saw the confusion on her friends faces. She grabbed a napkin and pulled a pen from an inside pocket of her leather vest. “I know, 'why did that happen?' Okay, so we've established that Equestrian minutes and hours are the same length as minutes and hours over here, right?”
“Right,” said Fluttershy.
Sunset began scribbling numbers on the napkin. “Okay, so, 20 hours in an Equestrian day, 200 days per Equestrian year. Multiply 20 by 200 and you get 4000 hours in an Equestrian year.”
She pulled out her phone and opened the calculator app. “Since we know hours are the same length on both sides of the portal, we can divide 4000 hours by 24 and that makes an Equestrian year correspond tooooo... 166.6666 repeating days over here. So just under six months on this side. Now, I was 26 when I came through the portal. Multiply 26 by 166.666 repeating, you get 4333.316 days. Divide that by 365, and you get just a skosh under 12 years here. Which is why when we go through the portal, we come out as adults on that side, but are teenagers on this side.”
“So you're saying that for every year that passes over here, just over two years passes in Equestria?” Fluttershy asked.
“The amount of actual time is the same,” Sunset clarified, “But because Equestria uses a different method of measurement, the calendar is different. And as a result of that, I can't use my Equestrian birthday as a basis for my birthday over here, because it would basically be in a different place on the calendar every year, and it would show up twice. I figured it was too confusing to bother with... and it wasn't like I had anybody who would have cared until the last couple of years anyway. And once we became friends, well, I just never mentioned it.”
“But... but... but we ALWAYS celebrate our birthdays!” Pinkie protested. “We just did Rarity's last week!”
“Look, it's okay, Pinkie. It's nobody's fault. I've never bothered to figure it out myself and I just used the day I came through as my own reference point to determine my age over here. It's not exact but...” Sunset shrugged. She looked back down at her phone. “Oh, jeez. I need to go. I'm meeting Flash at the movies. Sorry to bail on you.”
“That's okay,” Fluttershy said as Sunset stood up. The redhead looked over at Pinkie who was sitting with a blank expression.
“Pinkie? You okay?”
“But... but... birthdays are important...” Pinkie mumbled.
Sunset and Fluttershy stared at her, then each other.
“Did I break her?” Sunset asked.
“I think she just needs some time to process,” Fluttershy answered. “Go meet Flash. We'll talk to you later.”
“If you're sure.” Sunset looked skeptical. Fluttershy stood up and gave her a hug.
“I'm sure. Go have fun. I'll text you when she comes out of it.”
“Okay, let me know. See you later.”
“Bye!”
After Sunset left, Fluttershy turned back to Pinkie and put a hand on her shoulder. “Pinkie? Are you okay?”
Pinkie's hand shot up and grasped Fluttershy's wrist with an iron grip, causing Fluttershy to “eep!” again. Pinkie looked into the other girl's eyes, the blue dark and cold as the sea when a storm was rolling in.
“Now,” she whispered huskily, “We break in to Sunset's apartment.”
When Sunset got home that afternoon, she opened her front door to find her small studio apartment filled with streamers, balloons, and her friends who shouted “HAPPY BIRTHDAY, SUNSET!”
Sunset stared at them then looked at Pinkie, confused. “I told you I don't know when my birthday is over here.”
Pinkie threw her arm around Sunset's shoulder. “Yeah, but you still deserve a day we can celebrate you being here. So we all decided today's that day!”
Sunset giggled. “That makes no sense!”
“Neither does your crazy pony calendar!” Pinkie replied. “So we can make your birthday whenever we want and today is it! Cake?”
Sunset took the proffered plate and a fork. “Can't argue with that logic!”
Pinkie let out a squee of delight. “All right, HIT IT!”
Rainbow Dash hit a button on her phone and raucous party music blared out fo the speakers of Sunset's sound system. After a few minutes there was a knock on the door.
“Oh, jeez,” Sunset said. “I hope that's not Miss Bookbinder complaining about the noise.”
“Oh, I'm sure it isn't,” said Fluttershy. “We told her we were throwing you a party tonight before we set everything up.”
“Oh, good.” Sunset looked at the card table that had been set up for the refreshments. “Why are there so many bottles of chocolate syrup and whipped cream? There's only a couple of tubs of ice cream here.”
“Ummm...”
“THE STRIPPERS ARE HERE!” Pinkie grabbed Sunsest's wrist and threw her into her desk chair. “YOU sit right there, young lady. The birthday girl's getting a lapdance!”
The next morning, Sunset awoke to find herself in a warm, happy (and naked) pile of three of the strippers. She peered around at the mass of male, female, and non-binary flesh surrounding her and noted the not-unpleasant ache in her thighs.
From downstairs, she heard a low moan and Fluttershy's voice drifted up. "Quiet, you'll wake them up. And I told you, we're not done until we each get twenty."
Best birthday ever! Sunset thought, before rolling over and going back to sleep.
If you disagree with my thoughts on the Equestrian calendar... I really don't care. 
EDIT: After comments made by PiMan (and dealing with a bruised ego), I made some alterations to Sunset's Equestrian age based on revised calculations. Also added one last gag which occurred to me.
Badly Drawn Turtle helpfully pointed out the original error pointed out by PiMan that I deleted in a fit of pique, but was unable to recall after I calmed down. The corrections to the math have been made. Once again, I apologize to PiMan for deleting his comments.
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I think your idea/headcanon for Equestria's calender is okay.
Genius, even.
Well done.
Torn between "go on...." and "oh myyyyy"
Yeah
No one argues with cake! Or strippers!
Damn. I hope Flutters got out some pent up feelings that night too. She seemed kinda thirsty earlier
I want to focus more on the analytical time difference bits, but you short circuited my brain with cute zany plot and sexy shenanigans.
what!! gross
Flutters is kinky!
I caught that near-triple "WHAT?!".
I knew the moment I saw the title that this was based around/inspired by your mathematically proven headcanon.
Also, Sunset's 32?! That's almost as old as I am.
This was great
I really liked the headcanon displayed here!
And that's why Sunset Shimmer swings both ways.
I've noticed crazier theories. Probably come up with one of my own. In any case, I applaud the thought that you put into this.
The calendar difference was interesting. Very nice. I was laughing a lot when Pinkie brought up strippers. XD That ending was hilarious. Great work.
Absolutely brilliant headcanon.
Brilliant :)
Perfect.
AWWW!
And here I was wanting a punch line of her birthday ending up on February 31
Our fandom? Nerdy? Oh say it aint so... I love it.
That and different maturation rates between ponies and humans, presumably. As Sunset noted, the same amount of time still passes.
I have several other comments on this proposed calendar, but this is clearly not a story where one should dig too deeply. Silly fun, even if it broke Pinkie a little.
Where the hell is Calendar Man?
Okay, this brightened my day.
I like the math on your equestrian calendar. At least it's a logical progression.
Strippers don’t really do that kind of thing. In fact unless they are in Nevada there are a staggering number of laws against it.
The only problem is that one would never be able to adjust to that large a difference. A 20 hour day would be horrid.
9873110
Don’t hate on a good time!
I she knows how long an equestrian day and year are and she (presumably) knows the date she was born in equestria, she just needs to check with princess Twilight what date it is now in equestria and she should be able to calculate her birthday pretty easily.
9874565 Yeah, I'm not sure why they didn't got with 12 hours/12 hours.
9874551 True, but on the other hand Sunset Shimmer.
9874830
I’m not saying it might not happen, I’d just say a responsible party planner would hire people in the right profession for desired result.
Wow. Your explanation for Sunset's age is actually pretty creative.
Plus, a love the silly/sexy stuff you threw into the batch as well.
Math Nerd Powers, Activate!
The Bold sentence is where the calculation goes awry; the Equestrian number of days is used instead of the Canterlot High equivalent. It's just been established that an Equestrian year corresponds to 166.6 repeating Canterlot High days, so the correct calculation is 22*166.6 repeating = 3666.6 repeating; divided by 365 gives just over 10 years. Similarly, if Sunset is 39 in Equestrian years, that puts her at a little under 18 years old, which I'm guessing is not your intended age for her in this story; if your intent was to put her at a bit over 21, her Equestrian age would be a little over 46.
9875129
17-18 is Sunset's intended age in this story.
PiMan already pointed this out, and I, in a fit of ego bruised pique, deleted our interactions over it. After I calmed down, I apologized to him and made the correction noted in the author's note.
9875134
A stripper party at 17? Curious. The first part was the important bit, though. Mathematically speaking.
...But it's still not mathematically correct. If she's 12 on the Canterlot High side when she first crosses over, then she was initially a little over 26 on the Equestrian side.
I had fun reading this. From Pinkie nearly breaking to the ending punchline. Good work all around!
9874922 Sounds like that is exactly what happened!
9873184
Yeah. From a unicorn's perspective, if you're reduced rutting clothes-fetishist ape monsters to get off, the question of what they are hiding under their clothes is sort of a minor issue!
Hmm. So...at what age would a human have to be to become a horribly decrepit old pony on the other side? Then again, since time passes at the same rate on either side of the portal, but the year length is different...but Sunset's an adult on the other side and a teenager in human world...uh...
What no pony rides?
This is in no way, shape, or form out of character.
9875859
I believe the implication is that whatever magic the portal uses works off is a bit wonkers on the calendar date conversion due to the difference in the length of the days.
30 y/o in pony years -> 30 y/o human target
30*200 days -> 6000 pony days
30*365(roughly) -> 10950 human days
The magic maybe goes something like:
30 y/o pony -> 30 y/o human but instead of using 10950 human days it used 6000 human days (the pony day number value) thus 16.438356164 years old human body.
I think that's what the author is going for.
The only way this actually kind of works (i think) is that ponies must age faster biologically than humans do to keep 1 pony day = 1 human day as far as body development goes, that is if the magic was "correct" the 1 pony day to 1 human day would work out. i.e. a 10 year old pony (aged 2000 days) would be as developed as a 10 year old human (aged 3650 days) if the magic didn't mess up during the conversion factor.
I based my equestrian calendar on the numbers 10 for smaller increments, 100 for larger.
9874551
Only if the money is exchanged for the sex.
For all we know, Fluttershy was talking about pancakes.
9877261
The ache between Sunset’s thighs indicates it doesn’t really matter what Shy is talking about.
9877265
Ah, but it does.
Strippers can have sex like any other consenting adult, as long as they aren't paid *for the sex*.
9877016
This is really complicated, huh?
9877270
While true it would be a remarkably bad idea to try to do so in this manner, the police would likely not take excuses that it was spontaneous and had nothing to do with the payment received given that they were hired to come to the party. Otherwise this would just be the defense given to the police in literally any case of being arrested for prostitution or solicitation of prostitution. Also in this case they would need to convince the police that Sunset managed to seduce people into an unplanned orgy.
9877536
You assume that anyone would find out about the act. I can guarantee this happens way more than you think despite the law saying they can’t.
9877536
I mean yeah, but shit happens and as long as everyone's cool they roll with it.
9877614
9877966
The sad thing is all of this conversation started when I posted saying that Pinkie should have hired prostitutes as opposed to strippers. Responding to comments has lead to this point where it makes it sound like I am protesting the concept or something.
9878131
I think they're just saying that you're overthinking things, and you should just enjoy the story for what it is.
9878248
Overthinking is regrettably my nature.