date

I am wondering if it would be more efficient to store the integer value that the php function time() returns when a transaction is inserted into the database and then when querying the database to get certain transactions just use basic math functions to get the certain transactions. For example in I wanted to get all the transactions from the last 31 days lets say there was 980026719 seconds since 1970 and the number of seconds from the last 31 days is 2678400 select from table where time_of_trans >(980026719-2678400) I know that the syntax isn't correct, I am just curious if this would be the most efficient way to do this? thanks Randy