From the MAKE Flickr pool
Flickr member macetech demos his ChronoDot high precision real-time clock –
The ChronoDot is essentially a breakout board for the DS3231 temperature compensated real time clock. Even in varying temperatures, the DS3231 will maintain accuracy within one minute per year. In comparison, the popular DS1307 and similar devices can drift several minutes per month in varying temperatures, even if the external crystal and tuning capacitors are correctly selected. The ChronoDot has an integrated CR2016 battery on the bottom, which should last at least 8 years.
The temperature drift on standard RTC chips can come as an unwelcome surprise for projects which move between multiple environments. Assembled boards are available here.