פירוש על עבודה זרה 17:3

If a “Tanna” a reciter of texts, forgets exactly how many years it has been since the Destruction of the Temple, he can ask a notary and then just add twenty. This is because notaries follow the secular dating which began 380 years before the destruction, when the Greeks began to rule. So for instance, if it’s 452 according to the secular dating, then it is 72 after the destruction of the Temple.