James Hartnett came across an older millivolt meter with a nixie tube display and sensibly kept it for further use. In doing so, he has used an Arduino board to create a simple clock. Using PWM digital outputs and and some resistors, the Arduino can create a voltage that is related to the actual time. For example, an output of 12.37 mV would be 12:37.
Instead of using an accurate real-time clock IC, James has used the simple Arduino time library. However it is interesting to read the trials of making this successful, and take heed of his notes with regards to calibration and resistor values. For more information, circuitry and notes visit James' page here.
When working with time and Arduino (or any other system with the I2C bus), consider using our super-accurate real-time clock module. Based on the DS3232, it has a temperature-controlled crystal oscillator for accurate time keeping, battery backup and 236 bytes of non-volatile memory for user data. For more information, see our modules page.