While I'm far from a Mendix expert, this looks like a different in the epoch that Java uses as compared to what Unix uses ... milliseconds vs seconds.
I just used the Community Commons DateTimeToLong of [%CurrentDateTime%] and get 1406741928887. Taking Community Commons LongToDateTime of that value returns 7/30/14 10:38 AM.
Taking Community Commons LongToDateTime of round(1406741928887 div 1000) results in 1/16/70 11:45 PM.
On a Linux machine issuing 'date +%s' a 10-15 minutes after those results, shows 1406742701 ... which is about 800 seconds greater than the original long divided by 1000.
So, I think that the Community Commons method is working properly with an epoch expressed in milliseconds.
I had the same problem with the LongToDateTimefrom CC. I received the epoch time1590649318 from Azure. When I put in 1590649318 I get out 1/19/1970, 10:50 AM. When I multiplied 1590649318 with 1000 I get the correct date time.
The java behind the LongToDateTime is very simple: new Date(value). This function expects an epoch time in milliseconds, but I gave the function an epoch time in seconds, so I need to multiply my epoch time with 1000 first.