Time Difference between Correspondence Sent Date

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Hi,

I am unable calculate time difference using dates from candidate tracking.  I am trying to show the days between two correspondences.  I can pull in the dates when each correspondence was sent  but cannot calculate the days between both dates - it gives me an empty block.

Either one of the formulas below provides me with the date that a correspondence was sent but any calculation between the dates returns nothing - not  Zero.  However, when I use the dates in any other way they work - for example when I calculate the difference between Correspondence sent date and current date.