Duckie58:
I am testing one of Jack's RTC boards. As he indicates, these are really neat RTC chips, with many features. The power up and power down logging is especially interesting for my data logging arduino's. My board has been running for ~10 days (Jack can correct me on the last time set as he did it before delivery) and I just checked the time against the NPT source. The error is 7 seconds slow (I have not trimmed it yet - I want to see the out of the box accuracy).
What you are looking for is included in the library Jack wrote, as an example sketch.
Duck
The specs on the crystals I used were 20ppm, but the three we've tried so far seem to be doing better than that, which is not something I can say of the DS1307s I have. Without any calibration, one is within 2ppm after 13 days, and another is within 5ppm after 6 days. Just sent the latter unit to tlharv this afternoon. Lost track of the numbers on the one that Duck has.