The common western year does not have a set amount of days and instead depends solely on astronomical movements than a consistent, unchanging amount of time for a year (and for each month). Since not every year has exactly 365 days, and months vary in the amount of days they have – that would mean that most (essentially all) people are actually celebrating our birthdays later than what our actual birthday is going by an **exactly** 365.0 day year (or 365.25).

Especially given that time is consistent – how would it measure against a consistent count of exactly 365 days.

For example – for someone born in 8/3/1996, or 9/21/1976, their birthday probably isn’t 9/21/2019 or 8/3/2019 is it? And how do hours + minutes you were born factor into this?

Also – what is the shortest year possible in the western calendar? Is it exactly 365, or 365 and a quarter of a day? and whats the oldest?

Thanks.

In: Mathematics

OK so…

The shortest possible year the way our calendar is set up is 365 days, the longest is 366 days. The actual length of time it takes to go around the sun is 365.2422 days. The leap day (February 29th) is added in every 4 years, except it’s skipped every 3 out of 4 centuries – 2000 had a leap year but 2100, 2200 and 2300 will not. So this averages out over a 400 year period to 365.2425 – still off by about 26 seconds a year but if all else fails they’ll just skip an extra leap day sometime in 1.5 million years if they really want to.

Technically the exact number of times you’ve been around the sun might be different from your calendar age by up to 24 hours depending on how far from a leap day you were born and how far from the next leap year we are now.

Yes. To help aid this explanation, I’m going to refer to the earth’s relative position around the sun, the earth completing one exact orbit around the sun as a “true year,” and your “true birth time” as the relative position of the earth in its orbit the moment you were born.

The reason this is possible is due to the fact that a true year is not exactly 365 days, nor exactly any number of whole days. You hit on this in your question. Leap years are our way of slowing down our calendar year to match true years. It works, but it takes 4 years to come full circle.

Example: let’s say you were born at exactly noon on March 15th in 2004, a leap year. In 2008, a leap year, at exactly noon, on march 15th, your birthday on a calendar would be the same as looking at the true year; the earth is in the same relative position in its orbit. But, let’s look at what happens in 2011 a non-leap year. In 2011, the earth’s relative position is 18 hours behind our calendar year; so, in 2011, your “true birth time” would actually be at 6 pm on March 14th. This is when you would have completed exactly 7 orbits around the sun. The true year and calendar year will coincide again every leap year.

When you think of it (and it sounds like you have), referencing a birth “date” to a relative astronomical position of the planet to it’s star is arbitrary. The sun is also moving relative to the center of the galaxy and all other stars, so in literal terms, depending on your reference point, there is no time since you were born that you were ever in the same “place”.

Age and birthdays are entirely social constructs. We celebrate birthdays at our convenience, not when the number of days passed since our birth is exactly divisible by the 365.2425 (the length of the year).

The Gregorian calendar year is either 365 or 366 days. Not 365.25.