Your timestamp appears to be an amount of milliseconds since the epoch (jan 1, 1970).
You could convtert that to a datetime in Mendix through:
For me that seems to give the correct answer: may 28, 2020 9:27:00 (UTC)
You can also use the LongToDateTime action in the CommunityCommons module to convert a Unix timestamp to a DateTime object. There is also DateTimeToLong if you need to go the other way.
Hope this helps