I just saw a commit flying past as a response to PHP bug #50155. While right now it is proper to define the Unix Epoch at "1970-01-01 00:00:00 UTC", UTC wasn't actually defined until 1972. So it would be more correct to define the Unix Epoch as "the number of seconds elapsed since midnight proleptic Coordinated Universal Time (UTC) of January 1, 1970, not counting leap seconds." (from Wikipedia). What the bit "not counting leap seconds" means, I've already explained before in Leap Seconds and What To Do With Them.

Similarly, PHP's internal calendar is the ISO 8601 calendar. This is a modification of the proleptic Gregorian calendar. The Gregorian calendar implements the current set of leap years every 4 years, but not every 100 years, but again every 400 years (to get to an average year length of 365.2425 days). Obviously this calendar is only in use since 1582 (some countries adopted it as late as the 1900s), so using days like 1066-10-14 in the Gregorian calendar makes little sense because that calendar didn't exist back then. Now, PHP's ISO 8601-based calendar even modifies the Gregorian calendar by including the year 0. The Gregorian calendar goes straight from -1 to 1 which is a pain to do proper date calculations with. Therefore the ISO 8601 calendar uses Astronomical year numbering.