The man page for the date command says that the %s option will give “seconds since 1970-01-01 00:00:00 UTC“. I had expected that everything that date did would give output in my time zone unless I requested otherwise.. But it seems that in this case the result is in UTC, and the same seems to be also true for most programs that log dates with the number of seconds.
In a quick google search for how to use a shell script to convert from the number of seconds since 1970-01-01 00:00:00 UTC to a more human readable format the only remotely useful result I found was the command date -d "1970-01-01 1212642879 sec", which in my timezone gives an error of ten hours (at the moment – it would give eleven hours in summer). The correct method is date -d "1970-01-01 1212642879 sec utc", you can verify this with the command date -d "1970-01-01 $(date +%s) sec utc" (which should give the same result as date).