The way leap years work is actually pretty crazy.

almost all anyone can tell you about them is that "Feb 29th is every four years" but there's a bit more to it than that.

The whole point is to balance the calendar with the actual progression of time, but adding in an extra day every four years overcorrects, so

every 100 years where you are supposed to add the extra day, you omit it. However, this causes a bit of an under correction, so

every 400 years, when you are supposed to omit the leap year day, you actually keep it in. This last happened in the year 2000 when nobody really blinked an eye when we had a leap year, because as far as 99% of people know, there's just a leap year every four years for some reason.

when following this procedure, true time will align with the calendar for tens of thousands of years