This is what Garmin support told me when I queried why my Venu watch did not update to the new time zone when I went on holiday
Thank you for contacting Garmin,
In order for the device to change time zone you need to connect to GPS by starting an activity as the GPS is not active 100% of the time. Syncing your device through the Garmin connect app or connecting to a wifi signal will also sort this for you.