Hi KUDOS,
I always keep the quartz units from broken or damaged
clocks. They give a one second pulse, they are only
small and they are very, very accurate.
Even if you have to buy one they are cheap, and easier
than trying to get one second intervals by dividing
down the main supply or by dividing down from a crystal.
They are mostly one and a half volt working, but thats
not a problem as they are made to be very low current.
The R-C system is not accurate enough for a clock, as
any error is cumulative.
You want to build a clock for your computer ?
Your computer has no clock ?
You will need more than a decade counter to run a clock
from a one second pulse.
There are plenty of chip circuits about to give you the
appropriate divisions,
unless you want your PC to do the work and output the
drives to the seven segment displays ...
Is this some kind of project or homework exercise to
make a computer program drive seven segment displays ?
A clock made like that would stop when you turn off
your computer.
Best of luck with it,
John