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This function returns d if d is not null. If it is null, it returns a 0-length duration instead. An undefined or empty field is "null." Null values don't work in calculations, which is where the Nz() function comes in handy. When Nz() finds a null, it sees it as a zero. So, if you want to perform calculations on a field that may include a null, use the Nz function.
Nz([Actual Duration]) + Nz([Actual Duration 1]) + Nz([Actual Duration 2]) returns the totals of values in all these duration fields.