Difference between two times in format (hh: mm: ss)

I have two times, let say:

t1= '05:34:01' ;
t2= '20:44:44' ;

I want two evaluate difference between these two times in same format. Like the result of above must come as

t3= t2 - t1;  // 15:10:43 

What is the way to do it?