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))

Make a custom function using the javascript Date function:

Tools > Script Editor... >

function FROM_UNIX_EPOCH(epoch_in_secs) {
  return new Date(epoch_in_secs * 1000);  // Convert to milliseconds
}

And then in a new column you can do =FROM_UNIX_EPOCH(A1)

now, 3 years later from the original post, you can make...

Tools > Script Editor... >

function millisToDate(timeInMillis){
    var yourDateFromMillis = new Date(timeInMillis);
    return yourDateFromMillis;
}

and use your new function in the cell...

=millisToDate(cell here)