`GDateTime` is available since GLib 2.26, we already require 2.32 so this should be fine.
In G-P there is no direct use of `strftime` and only the PoHelper plugin uses `utils_get_date_time` with a fixed format (including `%z` which produced the wrong output on Windows until now!).
I'm not sure whether it is clever to use the GTK docs URL in dialogs, they get quite wide because of the long string. Maybe it is enough to refer to *our* manual where the GTK docs link can be found anyway?