parsing epoch timestamp to date

To get current unix timestamp we can do
$ date +%s
But how do we parse a file which contain epoch timestamp to date?
$ echo 1471365644 | perl -pe 's/(\d+)/localtime($1)/e'
if we have it in milliseconds, we could remove the milliseconds part with
$ echo 1471365644000 | cut -c -10 | perl -pe 's/(\d+)/localtime($1)/e'
Tue Aug 16 18:40:44 2016
Assuming your epoch seconds is 10 character. But if you have more or less, you need to do some other string processing first