Right, you haven't given us a lot to go on here so I'm just going to dive in with a little bit of code (in the hope that you'll do the same) and see what you think.
$date='2004-10-12 12:34:00';
$sql="INSERT INTO table (field) VALUES (ADDTIME('$date', '5:0:0'))";n //adds 5 hours
Is that what you're after?