This is how I save the pref:

		$today = date("F j, Y, g:i a");
		$pref['cs_timeupdated']=$today;
		save_prefs();

Try to get time and add minutes:

		//Time now
		$today = date("F j, Y, g:i a");

		//Time last updated
		$lastupdate = $pref['cs_timeupdated'];

		//Last update time + Update Time
		$time = $pref['cs_timelimit'] + $lastupdate;

	echo $time;

	if($pref['cs_timeupdated'] < $time){