Change the date and time on iPhone
The date and time visible on the Lock Screen are set automatically based on your location. You can also adjust them—for example, when you’re traveling.
Go to Settings > General > Date & Time.
Turn on any of the following:
Set Automatically: iPhone gets the correct time over the network and updates it for the time zone you’re in. Some networks don’t support network time, so in some countries or regions, iPhone may not be able to automatically determine the local time.
24-Hour Time: (not available in all countries or regions) iPhone displays the hours from 0 to 23.
To change the default date and time, turn off Set Automatically, then change the date and time displayed.
Tip: To change the font color and style of the clock on the Lock Screen, see Create a custom iPhone Lock Screen.