There are a few standard timezones missing from the Insert Date / Time feature that we use fairly regularly. Primarily IST (Indian Standard Time), JST (Japan Standard Time, although Japan is an option), and KST (Korean Standard Time).
It would be nice to have those as options for our community members in those timezones.
I have added a fix for this in
The reason why some of these timezone names aren’t available is a little complicated. We use Moment.js for dates, and Moment’s timezone list comes from the IANA time zone identifier list, which returns Asia/Kolkata for Indian Standard Time. Moment also includes aliases, but the full list of aliases is very long, so for now I have added aliases manually for the requested timezones.