 <p>One thing to consider is how many date calculations you are performing and where in the query your conversion is taking place. If you are searching a DB of 10 million records and you are converting a DateTime field into a Unix Timestamp inside of a WHERE clause for every single record and only ending up with 100 records in the query result it would be less efficient to use SQL to perform that conversion on 10 million records than it would be to use PHP to convert the DateTime object into a Timestamp on only the resulting 100 records.</p> <p>Granted, only the result of 100 records would be converted anyway if you put the conversion in the select statement so it would be pretty much the same.</p>