This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
If the year started on, for example, 1 March, two months later than
our present year, when was the leap day inserted?
[The following information is to the best of my knowledge true. If
anyone can confirm or refute it, please let me know.]
When it comes to determining if a year is a leap year, since AD 8 the
Julian calendar has always had 48 months between two leap days. So, in
a country using a year starting on 1 March, 1439 would have been a
leap year, because their February 1439 would correspond to February
1440 in the January-based reckoning.