Conditionally format cell with date output from user-defined function if not equal to toda

FileDateTime() will return the date AND time of the last save of the file. Perhaps you want to return just the date? In which case, you'd need:

Function LastSaveDateEx(Optional VolatileParameter As Variant)
LastSaveDateEx = Int(FileDateTime("C:\Filepath\example workbook.xlsx"))
End Function

Then, let's say your date is in A1. Select A1 and add a new conditional format based on a formula of:

=A1<>TODAY()

Choose to highlight in red when the formula is true.

That should be enough.

WBD