Another way to put it is that we don't number calendar years the way we do birthdays/ages.¹ Which probably accounts for some of the popular confusion here. You

Another way to put it is that we don't number calendar years the way we do birthdays/ages.¹ Which probably accounts for some of the popular confusion here. Your 23rd birthday is at the end of your 23rd year, and you're "23" from then until the 24th birthday. But you're actually progressing through your 24th year at that point. Conversely, during the year called "2000" we were in the 2000th year. We completed 2000 years at the end of it. Upon which we started marking progress in the 2001st year. --- ¹Gregorian calendar/Western-style birthdays