[eluser]Evil Wizard[/eluser]
check the the post values correspond to how your database accepts date/time fields, i.e. MySQL stores datetime in the following format "YYYY-MM-DD HH:MM:SS" if the format of the posted input does not match the structure it will convert the posted value to a datetime, which it usually gets wrong and magically picks midnight January 1st 1970 (the unix epoch).