Epoch time is Unix timestamp which counts the number of seconds since January 1 1970 00:00:00 UTC. Without getting into long discussion, follow is how your convert between human readable date and epoch time.
Convert Human Readable Date to Epoch Time
$now = strtotime('2010-04-25'); print $now;
Convert Epoch Time to Human Readable Time
$human_date=date("Y-m-d", 1251627495); print human_date;