This is the first C10 clock which I made back in 2011. It wasn't very accurate though, because it was built around a standard clock and the counting and timing corrections were in software.
C10 tick is 0.0864s, which translated into frequency is the odd 11.574(074) Hz. There is no such crystal on the market, and I didn't want to order a custom one, but fortunately there are many ways to achieve this exact frequency by using standard frequency crystals:
... and so on.
In my current schematic I use 100kHz clock divided by 8640. The reason for this is purely to increase the accuracy after division.
The best accuracy in general can be achieved by using a 19.2MHz crystal (with divider 1658880) as these come in the lowest frequency deviation and lowest temperature drift options on the market. But such high input frequency will significantly increase the power consumption in standby as well, so I decided to make a compromise on that for this demo.