A New Decade
There is disagreement as to whether the third 10-year decade of this century began last year, 2020, or begins this year, 2021. That is, some believe the new decade runs from 2020 through 2029, while others believe the new decade runs from 2021 through 2030.
The reasoning of those who feel that the next historical decade begins in 2021 is described in these two (among several other) articles:
Here are two of their main points:
Before addressing these points, it is important to understand that the terms “1st year” and “year 1” do not mean the same thing. For instance, when a baby is in its 1st year of life, it is not yet 1 year old until reaching its 1st birthday. Technically, since ages are measured in years, which are whole numbers, then a baby is 0 years old during the 1st year after its birth. Even when the baby is 6 months old or ½-year old, he or she still is 0 years old, since years are measured in whole number integers.
So prior to its 1st birthday, a baby is 0 years old; only on its 1st birthday is the baby 1 year old. It is considered to be 1 year old (or age 1) during the entire next year, until its 2nd birthday is reached, when it becomes 2 years old. Then for the next year, the 3rd year of its life, it is considered to be 2 years old until its 3rd birthday comes around.
Similary, when counting the number of seconds it takes for something to occur, we need to begin with 0, not with 1:
Basing the time count of the centuries and millennia on the birth of Jesus complicates things, since the exact timing of His birth is unknown. Therefore, ages of time based on the terms “A.D.” (Anno Domini or the Year of Our Lord) and “B.C.” (Before Christ) are mere estimates, at best. Thus, the terms “C.E.” (Common Era) and “B.C.E.” (Before the Common Era) are better terms to use for the Gregorian calendar, used by most of the world today, which followed the less accurate Julian calendar.
The beginning of the Common Era can be considered to be point 0. Every interval of time—whether a second, a minute, an hour, a year, a decade, a century, a millennia, or whatever span of time—begins with an initial moment, which can be called point 0. Until point 1 is reached, then 0 units of that time frame have passed.
Thus, the 1st year of the Common Era (C.E.) began at point 0 and ended at point 1. At point 1, and for the whole next year, the era was 1 year old, even though this was during the 2nd year. Then, for the whole next year, the era was 2 years old, even though this was during the 3rd year. During the 10th year, the era was 9 years old; it became 10 years (or 1 decade) old only when point 10 or its “10th birthday” had been reached, thereby ending the first decade. At that point, the second decade began.
Therefore, the age of an era, in years, is determined on the first day of any given year. Whatever the number that year is, that is the age of the era so far. For instance, at midnight completing the year 1999 and beginning the year 2000, the Common Era was 2,000 years old. At that point, the second millennium of time had ended, and the first decade of the next millennium had begun:
The first decade of the third millennium ended at midnight completing the year 2009 and beginning the year 2010, and at that point the second decade had begun:
Likewise, the second decade of the third millennium ended at midnight completing the year 2019 and beginning the year 2020, and at that point the third decade had begun: