AFAIK there is no firm convention. Feb is more natural ("My birthday is in February"), Mar is more logical (the 60th days of the year).
Anyway, I'm not sure there's any more or less logical date to use and Feb 29th babies seem to choose both about equally:
> “I love when people ask me, ‘Do you celebrate on Feb. 28 or March 1?’” said Raenell Dawn, a co-founder of Honor Society of Leap Year Day Babies. “I get to tell them, ‘Both, because I can.’ But I’m a February baby; I was not born in March.” An informal poll of the society’s members showed about a 50-50 split between the two dates, said Ms. Dawn, who is celebrating her “Sweeter 16” by turning 64 this year.
https://www.nytimes.com/2024/02/28/style/leap-year-explained...
If you want to celebrate your birthday at the moment earth reaches the same spot around the sun as when you were born, then we’d all have the same issue - we’d have to celebrate it 6 hours later every year, reseting every 4 years.
February 29 is very much a new day… which simply doesn’t exist on non-leap years.
If we're being scientific I guess all of us should move our birthday up by 1 day on leap years, but that seems annoying and not sure anyone really cares enough to.
Nasa explains:
365 +0.25 - 0.01 + 0.0025 - 0.00025 = 365.24225
Wait, is this right? omitting every 100 and adding every 400 should be enough.
EDIT: the 4000-year rule is just a proposal for now. So 365.2425 is correct.
const { format } = Intl.DateTimeFormat("en-UK", { month: "short", day: "numeric" });
const date = new Date(2024, 1, 29);
console.log(format(date));
// 29 Feb
date.setYear(2025);
console.log(format(date));
// 1 Mar
const year = 1000 * 60 * 60 * 24 * 365.2425;
console.log(format(new Date(2024, 1, 29).getTime() + year));
// 28 Feb
"Feb 29 + 1 year" is verbatim restatement.
---
You can say Feb 29 + 365 days = Feb 28. (And Feb 29 + 730 days = Feb 27.)
---
EDIT:
Note that in the context of birthdays, people use "calendar years," not "unit of time which is ~1 revolution around the sun."
Birthdays aren't celebrated every X million seconds after the moment of birth.
They are celebrated the same day each calendar year -- notwithstanding the fuzzy concept of "same day" for incongruent calendar years. There's no singuar right answer, but that is the core question: "what is the same day next (calendar) year?"
Correct.
> And Feb 29 + 730 days = Feb 27
Incorrect. It's Feb 28 again. In a normal, non-leap year, if you add 365 days then you get back to the same date.
As a surprise for a friend of mine, I threw him a gigasecond party on the day he turned 1000 million seconds old. Yes he was surprised, and a good time was had by all.
When I wrote "+ 1 year" I meant year to represent 365 or 365.25 days, not "the same day next year.":
>>> from datetime import date, timedelta
>>> year = timedelta(days=365.25)
>>> date(2024, 2, 29) + year
datetime.date(2025, 2, 28)
> Note that in the context of birthdays, people use "calendar years," not "unit of time which is ~1 revolution around the sun."
You've never heard someone say they've celebrated another trip around the Sun? I literally have a photo from 2007 of my daughter in Montessori celebrating her birthday by holding a globe and walking around a candle representing the Sun. I think most people probably don't really think about whether it's a calendar year or astronomical year because for most people, they are usually equivalent and it doesn't matter.
But that's all beside the point. All I meant to point out is that I disagree that March is more logical. I don't think logic points us to one month or the other. You may feel March is more logical, but I don't accept your priors, so for me March is not more logical than February.
Your calendar year birthday would be March 1 since one full calendar year has elapsed.
But Feb 28 is the nearest day to one full solar year, assuming you were born before about 6pm local time on Feb 29.
Two-day long birthday it is! :)
If you are born on 29th, on future 28th you are considered "too young", regardless whether a 29th exists or not. On future March 1st you are "old enough" again regardless.
If a 29th exists you are old enough already on that date. Drinking beer in Germany at 16, I guess in some countries at 20 could be relevant cases. For the more common minimum age of 18 for many things, the limit is reached always on March, 1st because a 29th cannot exist.
Tell us about your last marriage?, lol