The first decade in the calendar was complete at the end of 10 CE, or 10 AD.
Which means, according to the Farmers' Almanac, that the end of this decade is December 31, 2020, not December 31, 2019. According to the astronomical dating system through which it measures time, the observatory stated that the new decade would begin on January 1, 2021.
We seem to have collectively determined that decades begin in years ending in zero and end on years ending in nine.
Happy new decade to you all