The problem with an alarm clock is that the chip is
completely contained. The only information coming out would
be seven segment displays - multiplexed. You would have to
convert that back to some usable form OR build a memory device
to store those multiplexed seven segment outputs and then
drive the other displays. Four digits times seven segments
plus AM/PM is 29 D flipflop latches and LED display drivers.

What type of time do you want to store? For example, if
time of day, then the alarm clock solution is better. AC
powered clocks synchronize with the 60 Hz power line - whose
accuracy is traceable to the National Bureau of Standards. To
avoid the complexity of same, the alarm clock would be a good
foundation to start construction. You would probably have to
beef up or supplement the alarm clock's power supply to power
your latching registers and two additional displays.

Another alternative is to start from scrap with a single
chip computer. Again, your timing crystal accuracy is just
another consideration. But a single chip computer affords you
great flexibility to enhance your design.

Daniel J Beardsall wrote:
Greetings all.

Assume for a moment that I have no knowledge of electronics (not true,
but I only know what I've picked up and that's rather patchy). I'm
looking to make a little addon for my PC and I need a little help. I'm
after making 3 time/date displays using 7-segment (numbers) and
10-segment (letters) LED displays. One of them should actually show the
correct time, the others I'd like to show other static times, preferably
with the option to change them occasionally. The latter parts should be
within my reach but the actual clock is a bit more of a problem, our
school was a bit crap and didn't actually teach us how to do that.

I'd rather make it from scratch but I'll butcher an alarm clock if I
have to.

The other thing is that it needs to be powered from the PC's power
supply, so it'll need to run on either 5V or 12V, with some form of
battery backup to power just the real clock when the machine is switched
off. If somehow it could be made to display the system's internal time
that would be wonderful, but not necessary.

Any help at all would be vastly appreciated.