Set invalid time values to nil, otherwise MySQL saves them as 00:00:00 which can not be resaved.
Set invalid time values to nil, otherwise MySQL saves them as 00:00:00 which can not be resaved.

git-svn-id: 5f77af0fd8ad127f450dd50e49bfdc1c1477d6c5@26 20afb1e0-9c0e-0410-9884-91ed27886737

nickstenning/validates_date_time,jviney/validates_date_time