Hi Blapto,
Its not a good idea to use an R-C time constant as a reference
for timekeeping.
You would really need a crystal controlled frequency.
The higher frequencies are a little better timekeepers, but even
low frequency crystal control oscillators are ok for clocks.
The thing with clocks is that any error is cumulative,
so even a slight error will build up fairly quickly.
R-C timing is slightly affected by many factors, temperature,
humidity, even barometric pressure could have an effect.
The simplest thing for a regular one second pulse that i have
ever found is the unit from an old quartz clock. They can
usually be found easily. Its usually the plastic parts that fail,
the electronics is usually fine. They are very accurate and use
very little current. A excellent starting point for a divider.
Best of luck with it, John