Nz() works on any data type although IsDate() is preferred for dates.
Nz(yourfield, returnWhenNull) The Nz() function has default "returnWhenNull" values for numbers (0) and strings (""). I don't know if there is a specific default for dates or what it might be. If anything, it would be 0 which equates to Dec 30, 1899.
When integral Access functions don't seem to work, the problem is usually a missing reference. Open any code module, then tools/references. Fix any missing references and compile. Fix any compile errors.