it is known for certain (from history and archeology) that Mt Vesuvius erupted in 79CE.
When lava from the region was recently dated, the result was about 70CE - showing an accurate result.
The abstract in
Science:
40Ar/39Ar Dating into the Historical Realm: Calibration Against Pliny the Younger. The full text is available from there (free registration required for non-subscribers).
That and other useful links in http://
EvC Forum: Radioisotope dating links and information -->
EvC Forum: Radioisotope dating links and information