It's been said by some that Roman Numerals
weren't Y0 compliant: they had no number for zero. However, there was no Year Zero
. And their calendar
wasn't measured from the supposed date of birth of some insignificant child
born in the province
However, some programs today are genuinely not Year Zero compliant: they believe that between 31st December, 1BCand 1st January, 1AD there's a full year. This isn't usually all that important: the Gregorian Calendar changes in the last four hundred years (it's impossible to put a finer point on it than that) mean that you could easily be a fortnight out at any point.
Time / Calendar problems are really thorny, caused by a large number of inconsistancies and changes throughout the world. For example, you have to take into account changes in Daylight Saving, Leap year problems, Leap seconds (occasionally), the slowing of Earth's rotation, the changing length of the year and month, precession of the stars... hence why most sensible people are starting to use UTC to co-ordinate internet activities. Which suits me down to the ground, living in GMT.