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(Date x, Date 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([Actual Finish Date], [Estimated Finish Date]) returns the value in the Actual Finish Date field, if it exists. If the Actual Finish Date field is empty, this formula returns the date in the Estimated Finish Date field.