Time to Fill where the starting date is the later of two date fields

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I have a request to create a new Time to Fill metric where the starting date needs to be the later of Latest Fully Approved Date or Latest Reactivation Date.  The ending date will always be Offer Accepted Date.

Is there some way to evaluate both dates and only include the later date -- all in one TIMESTAMPDIFF formula?  And how would I account for the fact that some reqs have no Latest Reactivation Date so it will be null?

Or if I calculate the TIMESTAMPDIFF of each pair of numbers in different columns (Fully Approved to Offer Accept in one column and then Reactivation to Offer Accept in the second column), how can I then show the smaller of those two integers in a third column?  Again I think I still need to account for the fact that Latest Reactivation Date could be null.