If you want to create a "rounded" time stamp, for example, to the nearest 15 minutes use this as a reference:
<?php
$round_numerator = 60 * 15 // 60 seconds per minute * 15 minutes equals 900 seconds
//$round_numerator = 60 * 60 or to the nearest hour
//$round_numerator = 60 * 60 * 24 or to the nearest day
// Calculate time to nearest 15 minutes!
$rounded_time = ( round ( time() / $round_numerator ) * $round_numerator );
//If it was 12:40 this would return the timestamp for 12:45;
//3:04, 3:00; etc.
?>