I looked at moment.js ... if I install that the times will be good out till 2022 ... I know all software dies but 4 years is a kinda short feeling

huh, moment references this API in some browsers. Works for me in electron console:

Intl.DateTimeFormat().resolvedOptions().timeZone
// => "Pacific/Auckland"

I might use that (with a greyed out UTC -12 = new Date().getTimezoneOffest() / 60 as a backup)