Time calculations for Word 2007 schedule

I know I've done this before, I just can't remember how. I have a schedule that needs to be printed monthly, based on sunset. So i have a table with 4 or 5 columns. I want to enter one value in each column (x) and have the rest of the value calculated automatically.

The calculations I need are as follows:

(x - 1 hour) rounded to the nearest 5 minutes

(x - 70 minutes)

(x - 40 minutes)

This doesn't seem to be so difficult - I just don't remember how to do it!

TIA