We name the years before Christ as in 2000 BC, 1500 BC, 600 BC, and so on. But how did those people at that time count their years? They couldn’t have used BC in any of their calendars. Or as time passed, did they even label or keep track of their years at all?
The scriptures themselves give us the dating system used in the ancient world. A national calendar was based on the current king’s reign. The book of Kings keeps track of 2 calendars simultaneously because it tracks 2 nations.
And it came to pass in the fourth year of king Hezekiah, which was the seventh year of Hoshea son of Elah king of Israel, that Shalmaneser king of Assyria came up against Samaria, and besieged it.
For Judah this siege happened in 4 AH (anno Hezekiah) and for Israel this same event happened in 7 AH (anno Hoshea). Jeremiah uses this calendar and dates his prophecies in terms of the king’s reign. The Book of Mormon uses a similar dating strategy even after switching over from monarchies, noting an event that occurred “in the first year of the reign of Alma in the judgment-seat”. Even this was really just a subset of a dynastic dating system. From this point on, everything was dated from “the first year of the reign of the judges over the people of Nephi” as can be seen much later.
Another dating system, popular in Intermediate periods when there isn’t a stable dynasty, is to start the calendar from some notable event – usually one which is foundational to the nation using it. A key event in Solomon’s reign cross-references the date, not off of his father David’s calendar, but from the time of the Exodus. “And it came to pass in the four hundred and eightieth year after the children of Israel were come out of the land of Egypt, in the fourth year of Solomon’s reign over Israel, in the month Zif, which is the second month, that he began to build the house of the Lord.” Similarly all Nephite dates could be traced back to their exodus, regardless of the calendar in vogue at the time.
In fact the Book of Mormon gives us 3 dating systems to ground the reader for when a new one was getting established. “And also an hundred years had passed away since the days of Mosiah” – this is the reign of judges reckoning – “… And six hundred and nine years had passed away since Lehi left Jerusalem” – this is since the founding of the nation – “And nine years had passed away from the time when the sign was given, which was spoken of by the prophets, that Christ should come into the world.” This would be the new calendar. Just like in the Old World, when Christianity gained national recognition and then transcended boundaries, this became the standard date for Christians in the New World. “Now the Nephites began to reckon their time from this period when the sign was given, or from the coming of Christ.”
We continue this tradition today by dating events from the birth of Christ (AD, Anno Domini – “in the year of our Lord” (note the similarity to the regnal years above)). For simplicity, anything that antedates this pivotal event is dated in terms of BC (Before Christ – no Latin?). At the time of the event, it would have be dated in a regnal fashion, or from some earlier pivotal event as shown above.