Today I lashed up a Tiny RTC module to the "Occam's Beacon" system...
The RTC uses the Maxim DS1307 chip and communicates with the Arduino over I2C, which it shares with the LCD display. There's a battery on the module to keep the clock ticking when the system is turned off.
I've arranged for the code to start the beacon cycle on the next "even" minute after power up - all that remains to be seen is how accurate the long-term time keeping is! If the clock drifts I can correct things as I've added the ability to sync to the "top of the minute" with a button press. The code is a "provisional" version, which I'm still testing - I'll publish when I'm satisfied that it is worthy of your consideration!
tick-tock de m0xpd