The value you get by taking the difference of two timestamps is not a timestamp, but an intervall - and that intervall cannot be inserted as just hours - a timestamp is a defined moment in time, not an intervall - as I mentioned here you are simply using the wrong type for that column.
Please stop spamming the monastery with your rubbish postings - read
I'm a bit fed up with your behaviour here - posting, multiple times, without content, in the wrong sections, creating new accounts to make the same mistakes again - you are misusing a wonderful place, and with that you are taking away from yourself the opportunity to learn something from all the wonderful, helpful poeple here - kind of sad...
An intellectual is someone whose mind watches itself.
-- Albert Camus