Where the differences are up to 31 days, you can use simple subtraction and average, with a custom number format like:
d" days, "h" hours & "m" minutes"
For anything more than a month, you'd probably do best to work with a simply numeric format that expresses the output in decimal days, rather than trying to get it to do days, hours & minutes.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]