In this perticular case the cell that is changing colors does not need
a formula in it.

If Cell B8 contains a date and you want Cell D8 to be green when Cell
B8 is +/- 7 days from the current date do the following:

Select Cell D8
Select Format - Conditional Formatting
Change "Cell Value Is" to "Formula Is"
Put in the following formula:
=AND($B$8<NOW()+7,$B$8NOW()-7)
Select the Format button
Select the Patterns tab
Select Green
Click OK
Click OK

Change Cell B8 to different dates and watch it change to green whenever
the date is +/- 7 days from the current day.