How to add time to Clock.currTime

[JohnnyK]

Hi All,
  I did search but I cannot find it anywhere.  All I want to do 
is add 300 seconds to the output of Clock.currTime.  So basically 
if Clock.currTime equals 2013-Oct-04 17:19:31.3338333 then I want 
to subtract 300 seconds from that to get the time that it was 300 
seconds ago.  In other languages I would convert the time value 
to seconds and subtract the 300 seconds then convert it back to 
what ever time value it was before. I don't know how to convert 
300 seconds to hnsecs and I have never actually heard of hnsecs 
before.  Anyway if someone could help I would appreciate it.