It seems there are big gaps in written records before 1000 AD. The church seems to be responsible for most written records and with many of the prophets living for hundreds of years how sure are we that they kept a accurate accounting of time. If they had a reason would it have been possible to change the year without a largely illiterate population even knowing.
Hi there -- this gets asked here pretty often. You may be interested in this section of our FAQ on Calendars and Timekeeping, and particularly in this answer from u/sunagainstgold.