It looks like you are storing a unix timestamp, which is the number of seconds since Unix' birthday of some time in 1970. This is a lousy way to do it, and if you have the gumption then you have a great opportunity to improve your data by converting that lousy way of storing a timestamp to a "datetime" type column. Guelphdad has a good tutorial
for doing so.
But if you don't want to do that then you'll want to grab the current unix timestamp when you insert a row in the table (
$currentTimestamp = date("U");