I've always been annoyed about how hard it is to convert seconds-since-epoch to strings. I've always been using "date -d '1970-01-01 + 1234 sec'", but as it turned out, that's wrong because it uses the wrong timezone. Luckily, there's a slick replacement:

$ date -d '@1234'
Do 1. Jan 01:20:34 CET 1970

The right version of the "long" version is:

$ date -d '1970-01-01 UTC + 1234 sec'
Do 1. Jan 01:20:34 CET 1970