Samsung
Change Region and Time Zone auf Samsung
The Time zone field on Samsung phones only becomes tappable once Automatic date and time is turned off. Samsung locks manual region and zone selection while the automatic sync is active, because the system pulls both values from the mobile network or GPS. This protects against mismatched clocks, but it also blocks you from setting a custom zone for travel, dual-SIM setups, or testing.
To unlock the option, go to Tap on Settings ➔ Tap on General management ➔ Tap on Date and time. Inside that screen, switch off Turn Off Automatic date and time to disable the automatic sync. Now tap Tap on Select time zone, choose Tap on Region and set the country or area you need. Confirm by setting the actual Time zone entry that matches your location.
Keep in mind that disabling automatic date and time also stops the clock from syncing with your carrier network. If you travel often or rely on network time for accuracy, switch the toggle back on once you finish adjusting the settings. For users who only need a temporary change, for example while testing an app that behaves differently by region, this manual method is faster than waiting for network sync to update.