Epoch & Unix Timestamp Conversion

https://community.quickbase.com/quickbase/topics/how-to-format-a-date-returned-from-api

That post has what you need, sort of. It has the JavaScript that can do this. I just double checked it the code does work on quickbase still so they are still using/loading that library. It isn't as straight forward as just throwing that in a formula field though.

The relevant code from there is below. I will try and update this in a bit with how you might use it but this should get you started since you now know you have to use JavaScript. 

> $.datepicker.formatDate("mm/dd/yy", new Date( parseInt("1435579444018" ,10)) );