While Mormon does not explicitly mention it, he is giving us the correlation of the calendars that have been used by his people. There were three reckoning counts. The current year count marked time from the abdication of Mosiah that established the reign of the judges. The next reckoning date was the departure from Jerusalem. For the believers, the newest count began with the signs of the birth of the Messiah. For Mormon, this is the count that he is using. It is quite doubtful that it was a widespread change at the time, however, as it would have been significant only to the believers, and enforceable only when the believers were in control of the government. Even though Mormon indicates that the count of years from the time of the sign began at this time, it is more likely from a historical perspective that it happened after the arrival of the Messiah in the New World, an event that rather clearly confirmed the birth of that Messiah.