In the EventCrux project, I developed a feature that automatically detects the user's system time zone using the browser’s Intl.DateTimeFormat() API. When the user opens the event creation screen, the application checks whether the detected time zone is supported by the platform.
If the detected time zone matches one of the allowed zones, it is automatically pre-selected in the “Time Zone” dropdown.
if the detected timezone is out of the selected timezones it shows the the timezone you selected and it warns the to select in us timezones