Based on a fix for a recent bug, Derick Rethans wanted to clear up something about the Unix Epoc's definition and how it relates to the UTC time zone.

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).

He talks about the calendar PHP uses internally and how dates predating it's use don't make much sense either (as well as the modified version PHP uses that includes "year zero").