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(Date/Time x, Date/Time 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([Timestamp of Request], [Date Created]) returns the value from the Timestamp of Request field, if it exists. If there is no value in that field, this formula returns the value from the Date Created field instead.