decuct a constans value from time

[HarryKlein]

Could anybody help me, how can I deduct a given value deduct from a time-value?
Let's say I would like to know how much is 12:00-2 hours or 12:00-120 minutes?

I know the datediff-function but it the 2nd parameter wouldn't be a date-value but a concrete number.
Thanks

[davsth]

All date times are a number of days since I think 1st Jan 1900, but the important thing is that 1 is a day

Subtract 2/24 or 1/12 if you prefer