"on" should not use any more power than "standby", as e-ink doesn't use power until it updates the screen. however, as the processor is still running, there is always some power being used in either mode. the only time no power is used, of course, is when it is completely switched off.
on my sony reader, the way the clock updates is when the screen updates, by changing the page, or any other means that causes the screen to change.
i notice that for the m92, if you go into the full screen clock by tapping on the clock on the upper right hand corner, it does update periodically.