Unless you are jailbroken there should be no large-font time appearing at the top of the screen, other than the lock screen, and the lock screen does not show the time in small print at the top. So your description does not fit, except for two possibilities (off the top of my head).
1) If you are jailbroken, then check any clock or screen related tweaks you've made.
2) Someone has played a joke on you, and changed your wallpaper to a picture with the time on it. Check Settings > Brightness & Wallpaper.
Otherwise, I wait for the requested screen shot to see what's going on. You'll need to make two more posts before the forum lets you attache a file, so look around and say hello. We're a friendly bunch.