The 0000 year is valid.
The ISO standard for "Representation of dates and times" says:
"Calendar years are numbered in ascending order according to the 
Gregorian calendar by values in the range [0000] to [9999]."

Reverted fix for 21789: DATETIME with 0000-00-00 11:22:33 should be invalid, but is accepted
as it's not a bug.

Fix for 19370: DateTime datatype in MySQL has two bugs in it
will be reverted during 4.1 -> 5.0 merging as it was pushed to the 
5.0 tree.