|I put 5/1/2009 in Sheet2!A1|
I put =Sheet2!A1 in Sheet1!A1 so it also shows 5/1/2009
I used Conditional Formatting on Sheet1!A1 as follows:
Formula is =IF(AND(A1-TODAY()>0,A1-TODAY()<30),TRUE,FALSE)
Sheet1!A1 turned Red because today's date (4/2/2009) is less than 30 days from 5/1/2009. It doesn't matter that the original date is on another sheet. The Conditional Formatting is based on the result of the formula in the cell - in this case, 5/1/2009.
I then changed Sheet2!A1 to be 6/1/2009, which caused Sheet1!A1 to display 6/1/2009 and the Red went away because today's date (4/2/2009) is not less than 30 days from 6/1/2009.
If I wanted to apply the same conditional formatting to a range (or group) of cells, I would select something like A1:A10 then enter same formula in the Conditional Formatting...Formula is...field and it would set the same Conditional Formatting for each cell, but update the formula (A1, A2, A3, etc.) to match each cell.