Fixes #17246: Date custom fields can't store dates pre-1970.

If a user is to use custom fields for "date of birth", they will always get invalid date. The reason is that the validation assumes only dates are valid when their value is greater than 0. Though strtotime() returns 0 for 1970-01-01, false for invalid values, and negative for values before 1970.

PHP versions before 5.1 used to return -1 for invalid values. But that is not longer the case.
http://us3.php.net/manual/en/function.strtotime.php