Replace a Cells contents after a value is entered

I have a formula that tests to see if a Date entered in a cell falls within a certain Week.

The formula is:
=IF(A2="","",IF(AND(A2>=(WE-6),A2<=WE),"","Invalid Date"))

Where "WE" equals a Week Ending Date previously selected on another sheet (ie: Week Ending: Feb 9, 2008).

If I place this formula in a cell it will put "Invalid Date" in it if I enter a date that is not within 7 days of the "WE" date in cell A2.

My question is can I create a Macro that will put "Invalid Date" in cell A2 after the user originally enters a date in cell A2 that is not within the 7 Days of the Week Ending Date?

I would like it to enter the "Invalid Date" text immediately and this needs to occur on several cells (ie: Range = A2:A200).

I can't seem to create an "IF/Then" statement that will work.

Thanks for any and all assistance.

My first thought was that you will have to do some fancy VBA programming.

However, try looking into the DATA > VALIDATION options.

This might not give exactly what you are after, but might point you in an alternative direction.
A bit of lateral thinking might prompt the desired user response from a slightly different excel response...

Mark.

Another possibility would be to use Conditional Formatting, turning the cell red if the input is not as expected;
But teh Data - Validation looks the best to me ( without VBA of course)