`pacman` uses local time of your system. Indeed it seems like an ommision.
And the code says /* Use ISO-8601 date format */ - which is not entirely true :)
You may consider opening a feature request, so pacman.conf would allow either an option to include TZ in the logs (good idea) or setting custom time format (worse idea, as this would bind `pacman` codebase to the specific time formatting function).
I suggested a patch that just adds the time-zone-offset to the timestamp. Anyone interested can find it here: https://lists.archlinux.org/pipermail/pacman-dev/2019-March/023219.html
--Florian
