So is this the 500th February 29th?

February 29, 2008 1:47 PM

[leapdayfilter] Curiousity strikes: How many February 29ths have there been since the Julian Reform in 45BC?

Some lazy research (read: wikipedia) tells me there were perhaps 13 leap days in the Julian calendar between 45BC and 1AD. Then, my lazy mathematics would have me believe there were another 485 between 1AD and 2000, plus 2004 plus today makes today the 500th February 29th, which would be kind of cool...but I think my math might be off a little bit, someplace. It's really of no consequence, but now that it's eating away at me, I'd really like to know. Any calendar experts care to weigh in?

posted by myrrh to grab bag (3 comments total)

I think we probably already missed the 500th Leap Day. Under the Julian Calendar, every year that was divisible by 4 was a leap year; it wasn't until the establishment of the Gregorian calendar that the every-four-years-except-every-hundred-years-except-every-four-hundred-years rule was established.

So by my calculations, there were 395 Leap Days between AD 1 and AD 1582, and 101 Leap Days between AD 1582 and AD 2000, for a total 496 Leap Days in the first two millennia instead of 485. The 500th Leap Day, if my calculations are correct, would have been in AD 1964.

BTW, this discrepancy of 10 or 11 days between your calculation and mine is precisely why the Gregorian calendar had to be set 10 days forward when it was adopted. In essence, the ancients were holding extra Leap Days, and so Pope Gregory had to take these extra days out of October 1582.
posted by Johnny Assay at 2:28 PM on February 29, 2008 [1 favorite]

Seems to me there would have been 497 to 500 February 29ths between 1AD and 2000, depending on when a particular country adopted the Gregorian calendar. Remember the whole "not a leap year in years divisible by 100 but not by 400" only applies to the Gregorian calendar, not the Julian. Add the BC ones and we're definitely beyond 500 by now.
posted by DevilsAdvocate at 2:32 PM on February 29, 2008

Initially Julian Leap years were every 3 years due to the ambiguity whether the 4 was inclusive. As per wikipedia, there is debate which early Julian leap years are right, and there is much confusion between 12 BC and 12 AD. Many propose that 4 AD is not included. All seem to suggest there are either 14 or 15 leap years between 45 BC and 12 AD.

So if we hypothesis 14 from 45 BC - 12 AD,
plus 392 from 13 AD - 1582,
plus 103 from 1583-2008,
that's 509 leap days.
posted by yeti at 3:37 PM on February 29, 2008