URGENT ::::::

I have a drop down box showing
0 minutes
5 minutes
10 minutes
1 hour
2 hours etc etc

i then have another drop down box with
00:00
00:30
01:00 etc etc

i want to be able to subtract the first amount of time from the second i know i will need a case statment on the first one saying something allong the lines of

select case me.blah.selectedValue
case 1 'remove 5 minutes from time

BLAH = my new time which should be 00:25

end select

what do i do to subtract time ive seen that datediff can handle time but when ever i try it buggers up