Given the calendar changes that were made over the centuries, and the fact there is still no certainty as to the exact start date (birth of Jesus) it's not something worth worrying about.
It's all a bit screwed up by the fact that a complete Earth orbit isn't an exact number of days as well. Leap years are a good fix, but there's still a small time creep in the calendar year.
Four digit rollovers however only occur every 1000 years, and there's only ever been one. 1999 to 2000. (You can't count 999 to 1000 as a four digit rollover unless you stick in a leading 0 and that's cheating, and anything BC doesn't count because much of that history is myth and legend and bad maths. Poor old Hippasus found that out.
)
So, in order to justify a good piss up at the millennium, I'm going with the four digit rollover. (Although I wont deny any old excuse for a good piss up is OK with me.)