Didn't I state the obvious that the clock if unpowered, keeps running for 10 years from its own li-ion battery (and the contents of the RAM are kept too, that's why it's called Non-Volatile) and whenever you power it externally (i.e. from the same source as the microcontroller), the battery gets recharged for another 10 years? :) Peter's solution has the minus of losing the clock state if you turn it off or somehow interrupt the program (unplug, replace battery, reset to upgrade software) etc. The external RTC keeps running no matter what, unless you intentionally stop the internal oscillator by writing to its SFR.