It would indeed be a trivial exercise if the date information were available as well as the time of day. However, there's no indication at all in the OP that the date information is present in the data set and that is why it has garnered responses which are as varied as they are many.

BTW, the URL you included gives a 404, perhaps you meant http://search.cpan.org/~drolsky/DateTime-1.34/lib/DateTime.pm#Datetime_Subtraction?

In reply to Re^2: Average start time handling midnight by hippo
in thread Average start time handling midnight by chrisjej

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