I need some advice on whether I should save unix timestamps in MySQL or should I create a datetime datatype and store it like YYYY-MM-DD HH:MM:SS

I'm trying to go with the most efficient way possible. Should I have php do the math for me or should I have the mysql query do the math for me. Can anyone shine some light on this?

Thanks

__________________
one outstanding employee does more and costs less than two adequate performers.