Most aspects of the Clock display on the iPhone aren't customizable, but Apple does make it possible for you to customize a few minor details. You can switch between using 12-hour and 24-hour time, and you can designate whether you want the iPhone to automatically adjust the day, date and time for you. Adjust these and other aspects of the Clock display in the Settings app.
Tap the "Settings" app on the iPhone's home screen.
Select "Date & Time."
Tap the "On/Off" toggle next to 24-Hour Time until it displays green, which indicates the On position, if you want the iPhone's clock to display 24-hour time. Alternatively, turn it off to display 12-hour time.
Tap the toggle to the right of Set Automatically to the On position so the iPhone automatically adjusts the day, date and time zone according to the location of the iPhone.
Tap the toggle to the right of Set Automatically to the Off position to set the information manually. Tap "Time Zone" and then enter a major city in the time zone you want to use in the search field. As you type the city, a list of potential matches appears beneath the search box. Tap the matching city if it appears. If the city you're looking for doesn't appear, enter nearby cities in the same time zone until one appears in the list of search results and then tap that option.
Tap the day, date and time where the text appears in blue type beneath the Time Zone field if you disabled the Set Automatically feature. Use the scroll wheels to set the day, date and time if you want to set it to something other than the day, date and time that corresponds with the time zone you selected manually.
Press the "Home" button to save your changes.
Information in this article applies to the iPhone 5S running iOS 7.1. Procedures may vary with other versions or products.