I have two date time fields that can be used. One will populate from time submitted, the other is an overwrite if needed. I am creating a third field that will be the published date/time. I tried a calculation of:

=if(variable2="",variable1,variable2)

Nothing is populating in the "published date/time" field. 

This is all from the same form.

In the monitor page, it populates the variable for the published date/time as "invalid date"