DateTime limitations

To avoid problems with 32bits OS [...], check that the dateTime can be converted to a timestamp.
On my environment, a date before 1901-01-01 - using ```$dateTime->format('U');``` - is set as 2037-xx-xx if it's before 1901, so the validator doesn't work (and doesn't complain).
Merge pull request #1 from bonswouar/patch-1

DateTime limitations