Uses DateTime class in PHP 5.2+ to support dates outside the 1970-2038 range. On earlier versions will still fall back to strtotime with the old 32-bit Unix timestamp range limitations.
Patch by rememberthedot -- https://bugzilla.wikimedia.org/attachment.cgi?id=5416
Added a couple quick parser test cases to confirm the new behavior. However I am seeing some annoyances with how input time zones are handled, so we'll want to clean that up at some point. :)
