Account Trigger:
Apex triggers allow for complex logic to retrieve and update theTimezonefield based onPostalCodeToTimezone__c.
Fast Field Updates Record-Triggered Flow:
A record-triggered flow in Fast Field Updates mode can fetch the necessary data fromPostalCodeToTimezone__cand update the Account.
Why Not Other Options?
A). Quick actions: Used for user interface updates, not for automation based on edits.
B). Approval process: Not relevant to dynamic field updates.
References:Record-Triggered Flow:https://help.salesforce.com/s/articleView?id=sf.flow_build_trigger.htm