Creating "Calculated Field" columns in SharePoint 2007

You can pretty much use the same VBA syntax for calculated fields as in Excel. Here is a site that provides many examples of common formulas that can be used in SharePoint when defining "Calculated Field" columns.
Here is a formula that I created at work:
=IF(YEAR([Due Date])=1899,"No Due Date",TEXT([Due Date],"MM/dd/yyyy"))
Blank dates in SharePoint are assigned a date in year 1899. The above formula displays a message "No Due Date" for invalid dates or the valid date using format MM/dd/yyyy.