Calculating the Date of Easter
It's even more "off" than that.
One of the biggest problems the Council of Nicaea faced regarding the dating of Easter was that the imperfection of the Julian Calendar had already thrown off the date of the Spring Equinox by about 4 days in the four centuries since it had been enacted. But rather than resetting the calendar (as they did about 1300 years later with the Gregorian reforms), the Council decided to simply redefine the dates of the equinoxes.
December 25 (the old Winter Equinox) was already becoming popular at the time for Jesus's "birthday;" but it wasn't yet officially established. By the time it became "officially" recognized as such (a few decades later), everyone had forgotten why the date was chosen in the first place (the Equinox), or even that the date wasn't even the original December 25th in the first place!