How to calculate time difference?

I have a table called report, and there is a column called time
the format of time is hh:mm, like 21:20
and i want to set up a query to calculate overtime
for example

Engineer time overtime
john 21:20 (this one to calculate
overtime, which is the time after 20:00)

what is the formula to calculate time?
i tried to type OT: [time]- 20:00 and it doesnt work.

so is there any method to calculate that?

thx all


--
One moment that for me epitomises my time here is when Steven Gerrard, my
new captain at the time, scored against Sofia and ran over to the touchline
to celebrate the goal with me and the staff. I want to stress again that
there is no bitterness; there is no ill feeling just a huge sadness at the
moment.