Temperature-compensated real-time clock with battery backup
The Chronodot by Macetech is an open source clock module that fits into the socket in the Bulbdial Clock kit. It's based around the Dallas DS3231S real time clock (RTC) (datasheet). The hybrid chip incorporates a temperature-compensated crystal oscillator (TCXO), and is specified to provide accuracy within ±2 minutes per year, if kept between 0 and 70°C. The Chronodot (V1) module comes almost ready to use, with the DS3231S pre-soldered on the board. You get to solder the headers and (included) lithium battery. Battery life is estimated to exceed seven years when used properly, and the battery is replaceable. The Chronodot has been released as an open source design as of September 2010.
NOTE: Due to component supply chain issues, Macetech is redesigning it with a slightly different form factor to be more friendly to breadboards, and the electronics should be 100% backwards compatible with the added benefit of an EEPROM. Coming in late July.