database value is not updating

[eluser]mehwish[/eluser]
hi can anyone tell me why it is happening when i put a variable in a database table (variable contains the string of this format 0000-00-00 00:00:00 but it is not a string like date month year and time is selected from calender and that selected date time is put in that variable ) it is still set to null i.e. 0000-00-00 00:00:00 whereas when i manually insert 2011:08:08 00:01:01 , the 0000-00-00 00:00:00 is replaced by that manual values. The database table field is of type timestamp.