Relating to my other post on Get unix epoch from DateTime in Logic Apps, there is a much simpler way to calculate UtcNow from a Unix timestamp. There is even no real math involved.
All you do is make use of the built in function addToTime.
Here is the expression:
So if you receive a JSON body with a tag called UnixTimeStamp, containing the Unix timestamp it will look like
Hope you can make use of it