You don’t need to announce it a decade in advance. Time zones change all the time, we already know how to handle it and it requires no global coordination. Some country announces that part of its territory is changing time zone, the tzdb is updated, and everything continues working fine for everyone. This happens a few times a year already.

Sometimes on quite short notice too. https://codeofmatt.com/on-the-timing-of-time-zone-changes