You are correct. The vast majority of people in the world do not really understand mathematics or actual "numbers" and their relationships. The problem gets even worse in the computation of "years." Throughout the years, time keeping methods have changed with change in dynasties, global events, Papal orders, etc.
In the modern world, most of us use the Gregorian calendar. In 1582, Pope Gregory XIII, issued a Papal Bull that took the world off of the Julian calendar used up to that time. (This discussion could go on forever, but space is a constraint here, so I'll skip over a lot!) Basically, it boiled down to the Roman Catholic church being uncomfortable with the ever advancing dates of Easter, because of the measurement in Julian dates. (Easter is tied to the vernal equinox.)
None of this takes into account the ancient Jewish calendar, since all of the Christian time keeping was based on the estimated time of the birth of Jesus. Compounding the whole mess is the fact that since most people don't fully understand numbers and how they relate to one another, no one ever starts counting by saying "zero, one, two, three, etc. So, in the case of decade and century counting, the decade and the century ends with "9 or 99" i.e 2009 or 1899, and begins with "0 or 00", i.e. 2010 or 1900. Looking back to the "Y2K" panic of the year 2000, it's all really very silly since time is a concept that only humans consider and we don't really know or agree how to keep track of it. (Y2K was off by more than three years depending on when the Gregorian shift occurred and if you considered "2000" as the beginning or end of a century.)