[PHP] subtracting time from date and time

[Richard Kurth]

I am trying to figure out what is the most accurate way to find the time
after I subtract 5 min,15 min, 30 min 1 hour 2 hours and 5 hours from a date
and time that is formatted like this 
20070617T193500 this is the way it has to be formatted to work in a
vcalendar