The easiest way to do this would be to leave your variable in the unix timestamp format.
$MyTime = time();
Then you can substract and add times to this value in increments of seconds:
$MyTime+=60; //increase by 60 seconds (1 minute)
$MyTime+=3600 //increase by 3600 seconds (1 hour)
...
When you want to output the formatted date, then call date():
echo date("Y-m-d H:i:s", $MyTime);
Brett