Anonymous Monk,
Have you looked at Time::Duration? I wrote a toy module for practice purposes that did something along this line but never released it as there was prior art.

Update: Perhaps my early morning brain didn't make clear what I was thinking when I wrote this. To get just what you want is rather straight forward as others have shown. I asked the question about a module not because I thought it was necessary for your problem, but perhaps because:

• You didn't know how easy it could be
• You didn't know it existed
• You might not need to come back for help if your requirements changed