Your tenth birthday is the end of your 9th year. You begin a 10th year.
A human lifespan isn't counted in the same way as the earths.
It has leap years and leap seconds.
18th century
1701-1800
19th century
1801-1900
The Twentieth Century of the Common Era began on January 1, 1901 and ended on December 31, 2000. according to the Gregorian calendar, (2000 was the first century leap year since 1600).
In the
Gregorian calendar, a
Century leap year is a year that is exactly divisible by 400 (and, thus, as with every other
leap year, qualifies for the
intercalation of February 29). The years 1600 and 2000, for example, were century leap years; the century years of 1700, 1800, and 1900 were not century leap years. The next century leap year will occur in 2400. Century leap years always start on a
Saturday, and the February 29 intercalation of such years is always a Tuesday.
The century year "divisible by 400" rule of the Gregorian calendar was considered an improvement over the previously utilized
Julian calendar which had provided for a leap year every four years; this practice resulted, over the centuries, in too many leap days being added to the calendar and placing it out of step with the astronomical seasons.