Apple gives you two choices for the Status Bar clock – 12-hour time or 24-hour time. That's it. If you want more choices, then a new free jailbreak tweak called StatusTime+
by iOS developer Luke Mitchell will let you change the format of the time in your Status Bar to your liking.
With the tweak installed, you can set your Status Bar time with nearly any formatting you want. You can make it show seconds, minutes, hours, days, months, years, and even custom text. There are characters to indicate 24-hour time, 12-hour time, and even to hide or show AM or PM. The tweak includes a full guide for formatting in the preferences pane from the Settings application, which is shown below:
Here, you can enable or disable the tweak on demand, choose to hide or show the time on the Lock Screen Status Bar, enter the format that you want the time to show in, and choose the time interval (minutes or seconds) that you want the time to refresh. You would only pick seconds if you added seconds to your Status Bar time so that you could see your seconds in real time, but take note that this will have an impact on battery performance.
Every time you make changes, you'll need to tap on the Save button to keep your changes, which are instant, so you won't need to respring to see your changes.
If you enable the time on the Lock Screen Status Bar, you'll want to use a Lock Screen customizing jailbreak tweak to hide the Notification Center grabber, otherwise, it'll get in the way of seeing your time, so this is something to keep in mind. Other that that, it seems to be a solid tweak for customizing your time format in the Status Bar. If you want to give StatusTime+ a try, it's available for free in Cydia's BigBoss repository for free.
iPhone 5 running iOS 7.0.4