By default, when users update the Time zone field in their Service Portal profile, it only changes their session-level preference. This change does not persist after logout because it doesn't update the value in the sys_user record.

• Users change their time zone in the profile page (/sp?id=user_profile)
• Change applies temporarily (session only)
• Time zone resets upon logout
• Users cannot update their permanent time zone setting in Service Portal

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There is an existing Time zone field on the service portal profile page (/sp?id=user_profile&sys_id=6816f79cc0a8016401c5a33be04be441) which when updated, just sets the session user preference and remains valid only for that particular user session. Logging out of that session will reset the Time zone value to the value present in the sys_user record.

However, if the requirement is to update the time zone as a whole which would last permanently, 'Time zone' field in "sys_user" record needs to be updated. However 'Time zone' field from the sys_user table is not available by default in Service Portal 'profile' page. We need to configure the service portal view and add this field onto the form.

Steps to configure portal view and add 'Time zone' field.

• As an admin, open 'Users' table and select a sys_user record.
• Right-click on the form banner (or) click the hamburger to select view.
• Go to the Service Portal view and then right-click header select configure form layout make sure you are in the Service Portal view, then select the timezone field on the available side and move to the selected sided and place where you want it on the form.
• Save
• Open the service portal and navigate to the profile page by clicking UserName --> Profile on the extreme right of portal menu. Now users should be able to see the 'Time zone' field under the 'About' section and updating this field would update the Time zone on sys_user record.

Refer to the below community article which explains the same.
How to change User time zone from Service Portal