Return the sum of date differences when grouped.

Hi all,

I want to get the sum of date differences between {custrecord_wevtlog_date_ended} and {created} grouped by employee ({custrecord_employee}).

Custom Record: Event Log
EventLog.custrecord_employee = "Foo Bar"
EventLog.created = "1/23/2017 1:51 pm"
EventLog.custrecord_wevtlog_date_ended = "1/23/2017 1:51 pm"

What I really need to be able to do in the result columns is SUM({custrecord_wevtlog_date_ended} - {created}) GROUP BY {custrecord_employee}.
I have tried a whole bunch of different combinations and end up with either an unexpected error being thrown or invalid expression errors.

SUM of TO_NUMBER({custrecord_wevtlog_date_ended} - {created}) ends up returning an empty string (I think it may be ignoring the time portion of the dates).