Tried this formula, but it doesn't work...

=IF(B2="", [A2], [B2])

I THOUGHT the formula above basically was saying, "if B2 is null then put
the A2 data in this field".

Trying to calulate dates for tasks...
Column A has the due date
Column B has the actual completion date
Column C should have the formula
Column D should have the results of column C minus column A

The idea is to show what tasks are late. For calculation purposes though
(and ease of use for the end user) if there is no "Actual Completion Date" in
the spreadsheet - in other words, if the task is still in progress, then I
just want the number to return as zero.