formatted 'time' data in calculations

[Ross]

There seems to be no shortage of information around on how to use the 
time module, for example to use time.ctime() and push it into strftime 
and get something nice out the other side, but I haven't found anything 
helpful in going the other way.

That is, given some formatted text describing times - is there something 
that makes it easy to calculate time differences, or do I have to index 
my way through the string pulling out characters, converting to integers 
etc...

Data is formatted:

    t1 = 09:12:10
    t2 = 11:22:14

I want to calculate tdiff = t2-t1

Any suggestions? (Thanks for anything you can offer)

-Ross