Conan Kelly wrote...
....
I opened a .WK4 file in Excel (I'm guessing that is a Lotus 1-2-3 file).

Correct.

I then tried to use the DATE(y,m,d) function. It worked okay as long as month
was 1-12 and day was 1-28, 1-30, or 1-31 (depending on the month).

But the "extended functionality" of DATE would not work like it does in a
new Excel workbook. Every time month or day was changed to a number outside
of those ranges, it would return #NUM!. I tried saving it as a Microsoft
Excel Workbook (1st choice), but it still behaves the same way.
....

WK4 files are treated as having Transition Formula Evaluation enabled.
123's @DATE gives the errors you describe, and in transition mode Excel
does the same. Disable transition formula evaluation by running the
menu command Tools Options, select the Transition tab, and uncheck
the box for Transition Formula Evaluation. Seems you may also want to
uncheck the box for Transition Formula Entry as well. Note: this may
cause different formulas/function calls the fail.