How can I change the date format printed in headers/footers?

You can pass a parameter to the "$D" token.  This parameter contains the same formatting options as for the C++ runtime function strftime (or take a look at GXIntlStrFTime in Objective Grid).

So, you could specify "$d{%m/%d/%y}" for MM/DD/Year

or "$d{%d/%m/%y}" for DD/MM/YEAR

or "$d{%y/%m/%d}" for YEAR/MM/DD

and so on.

Example:

GetParam( ).>GetProperties( )->GetDataFooter( ).StoreStyleRowCol(1,1, "$d{%d/%m/%y}");