CLONE -isValueModified returns true for timestamps that appear different but are equal

This is a copy of DC57:

"Doctrine_Record::isValueModified compares timestamps as strings and values that are equal such as 2009-09-02 00:00:00 and 2009-09-02 are treated as modified.

The code should do a second compare on the strtotime value if the strings appear to be unequal."

The changes made with this ticket causes problems with old timestamps because of strtotimes behaviour with dates before beginning of unix-period. I attached a diff for the original testcase to demonstrate the problem.