As long as Now() is greater than the time  you wish to deduct, you can
simply refer to the cells concerned.

ie: the source cell d1 contains your 15:25:00
the formula will be =now()-D1
You will need to ensure that the destination cell is formatted to the
required time format.

If however, you are spanning midnight you will need to adapt the formula
to:
=if(Now()>D1, Now()-D1, Now()+1-D1)
effectively the +1 is adding 1 day/24 hours.