The clock could be displayed in the top row of the screen or in the banner area. Not sure, which area you are referred to... but try to look for a kind of "Display" option in the Set Date and Time settings:
(you can only access Set Date and Time with Airplane Mode enabled)
1. From Set Date and Time, touch Time.
2. Enter the time, then touch Done.
3. Touch Date.
4. Enter the date, then touch Done.
5. Touch Set to save.