Fast field updates record-triggered flow and account trigger are two automation tools that can be used to implement the feature of updating the Timezone field based on the postal code. A fast field update is a type of flow action that can update one or more fields on a record without using any Apex code1. A record-triggered flow can be configured to run when a record is created or updated, and can perform actions such as fast field updates, invocable actions, or subflows2. An account trigger is a piece of Apex code that can execute before or after an account record is inserted, updated, deleted, or undeleted, and can perform complex logic or callouts to external services3. Both of these tools can access the PostalCodeToTimezone_c custom object and update the Timezone field accordingly. References:
* Fast Field Updates, Trailhead module
* Record-Triggered Automation, Trailhead module
* Apex Triggers, Trailhead module