The daily email message list had a copy of my lost message so I'll paste it below:
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Sounds easy. Its little more than an alarm clock by design.

The external controls "with manual override" requires some more
circuit information.

The clocking modes, actual time and presentation time, suggest that you'll need
to separate the clock function from the rest of your code so that you can
switch sources and let most ofthe code run the same.

This is done by turning the clock interface into a logical interface. Instead
of reading a real-time clock (RTC) chip directly, you'd load a timestamp into
RAM and make your routines access the RAM image. This allows to the synthesize
the RTC RAM data in presentaton mode.

If you aren't using a RTC chip but are using a timing interrupt in the 8051 to
keep time, this is easier though less timing precise. All you'd have to do to
switch between live and presention mode is switch the interrupt initialization
routine and probably a countdown section in that code that makes the difference
between minutes and hours.

aka J