Update: This feature was removed in iOS 10, and you only have a digital clock now. Miss this feature? Let Apple know.
If you like a digital clock better than an analog clock, and are using the iPhone built-in clock/alarm/timer app, then just tap on the analog clock face to toggle to a digital readout. Tap again to toggle back to the analog clock face. iPad doesn't have this feature as both analog and digital are displayed at once using the awesome map of the Earth. This is available in all recent versions of iOS and in the upcoming release of iOS 9.