Calculating timespan

[Erhard]

I've been looking at the date/time classes and I'm at a loss as to how 
to do this (probably too used to other platforms).

I have two date/time values. One represents 'now' and the other the last 
modified time of a file on disk (from stat). I need to calculate the 
difference in time (i.e., a 'timespan') between the two so I can tell if 
the file has been modified in the past X minutes and do something to it.

Thanks =)