These are needed for the calendar widget, and likely others as well.
These two dependencies are 6MiB when taken together, although they're likely to already be installed on many (but not all) systems (for GNOME, the dependency chain is gnome -> gnome-settings-daemon (and a few others) -> libgweather4 -> tzdata, gnome -> gnome-initial-setup -> iso-codes), so it's not too big of a deal. For the systems that wouldn't otherwise have it, it's better to be explicit.
Split off from !31986 which is taking far too long to complete.