I think I can imagine how such a system came out to be, until fairly recently keeping accurate timekeeping and even datekeeping might have been difficult, espec

I think I can imagine how such a system came out to be, until fairly recently keeping accurate timekeeping and even datekeeping might have been difficult, especially for isolated people. You may not have known the exact date of one's birth, but you know that it was "3 winters ago" or somesuch. You wouldn't know the precise day of one's birth, but you'd know the year and use that instead.