microcontroller - RTC Clock Calibration -

i looking cost x performance method calibrate microcontrollers rtc clock reference (usually 32.678khz).

i need way find reference clock drift , inform microcontroller, can calculate , aplly correction values.

i found these interesting methods described atmel. item 2.2.2 indirect measurement using 1hz reference signal seems adequate far. still open other approaches, though.

thank in advance.