The simplest way, not requiring any JS programming, would be through a formula, dividing by 86400 seconds per day and adding to January 1, 1970. For example the following gives 21 July 2017:
=1500598288 / 86400 + DATE(1970, 1, 1)
To convert a whole column of numbers, just use ARRAYFORMULA:
=ARRAYFORMULA(A:A / 86400 + DATE(1970, 1, 1))