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Crystal Oscillator

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Circuits that need accurate timing require a very accurate oscillator.

An RC astable circuit based on NAND, NOR or NOT gates tends to be unstable.
Temperature and power supply voltage variations cause the oscillator frequency to change.
A 555 timer circuit will usually be more stable than the circuit above but still not good enough for a digital clock.
A crystal oscillator will be very stable and minimally affected by power supply and temperature changes.
It is even possible to use a temperature controlled oven for the ultimate in stability.
The CARRIER GENERATOR in a transmitter often uses a quartz crystal.
For superb frequency control, the oscillator can derive its frequency from highly accurate GPS satellite transmissions.

The circuit below is based on a CMOS 4000 series NOT gate. The UB in the part number indicates "unbuffered". To get pulses once per second, several divide by 16 counter chips can be used.

32kHz Oscillator

If less accuracy is needed, the trimmer and 3.3 pF capacitor can be replaced with a fixed 15 pF capacitor .