Answer by Dharmendra Soni for Getting the client's time zone (and offset) in JavaScript

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat/DateTimeFormat

The Intl.DateTimeFormat() constructor creates Intl.DateTimeFormat objects that enable language-sensitive date and time formatting.

Intl.DateTimeFormat().resolvedOptions().timeZone // Asia/Kolkata