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(TimeOfDay x, TimeOfDay y)
This function returns x if x is not null. If it is null, it returns the alternate value y instead. Null means that a field's value is undefined. In other words, no one has entered any data in that particular field. It's empty. Its value is null.
Nz([Finish Time], [Closing Time]) returns the value in the Finish Time field, if it exists. If there is no value in the Finish Time field, this formula returns the value in the Closing Time field.