New decade from January 1, 2020 or January 1, 2021?

How to determine the first year and the last year of a decade (decade) is still a controversial topic in the world for a long time.

Many people believe that after the end of the second decade of the 21st century, the third decade will start on January 1, 2020 and end on December 31, 2029. For others, the new decade is counted from January 1, 2021 and ends on December 31, 2030.

New decade from January 1, 2020 or January 1, 2021?
Pope Gregory XIII and the revised calendar of 1582 – (Image:

Which of the two “schools” above is correct? Should we welcome the new decade on January 1, 2020 or wait until January 1 next year? And why this nagging controversy?

To find the answer to this question, the New York Times (USA) interviewed leading scientists at the United States Naval Observatory (USNO). USNO is the place to manage and operate the “master clock” atomic clock to determine the time for the whole United States, for the military and the GPS global positioning satellite system.

The view of scientists at USNO is that the new decade will begin on January 1, 2021. The reason is that the USNO uses the Modified Julian Date (MJD) numbering system to measure time.

The MJD calendar system is also used by astronomers and surveyors worldwide because of its mathematical and temporal accuracy (which has been precisely recalculated by scientists).

People have known how to count time since ancient times, based on the number of days the Moon rotates around the Earth to calculate the year, now called the lunar year . According to this calendar, a year has 354 days, divided into 12 months. Since 354 days is less than the actual number of 365 days of Earth’s one revolution around the Sun, there are lunar years that add a leap month to 13 months.

The Egyptians have been using the lunar calendar since 4236 BC (or “Before Christmas” ) and the Chinese since 2637 BC. According to a 2013 archaeological find, the ancient Scots began using the calendar as early as 8,000 BC.

The solar calendar system that the whole world is using today has its roots in the ancient Roman calendar system . In 45 BC, Roman Emperor Julius Caesar decided to improve the old calendar with the help of leading Greek astronomers and mathematicians of the time.

The new calendar was called the Julian calendar – derived from Caesar’s Julius name, and came into effect in the Roman Empire on January 1, 709 according to the old Roman calendar.

New decade from January 1, 2020 or January 1, 2021?
Emperor Julius Caesar, who is credited with applying the Julian calendar widely used today – (Photo:

The Julian calendar calculates that the Earth revolves around the Sun in 365 days and 6 hours, so a year is equal to 365.25 days, but the correct time is actually less than 11 minutes, only 365 days 5 hours 49 minutes 12 seconds ( 365.2425 days). Thus, the longer the Julian calendar is, the more deviant in real time.

In 525, a monk named Dionysius Exiguus (470 – 544) modified the Julian calendar system to more accurately determine the date of Easter. He was the one who devised a way to use Roman numerals to order the years.

Accordingly, the first year according to the numbering of Exiguus is the year of the Nativity called the year of Anno Domini (abbreviated AD), these two Latin words mean “year of our Lord” . At that time, because the Roman numeral system did not have zero (0), Exiguus used the name of the first year as the year “nulla”, the Latin word meaning “nothing”.

The number 0 and the decimal number system only appeared in Europe in the 13th century thanks to the European-Arab cultural-scientific exchange. According to many researchers, the number 0 was invented by Indian mathematicians in 458 BC, then spread to the Arab countries of the Middle East, then to Europe and later the whole world.

New decade from January 1, 2020 or January 1, 2021?
Monk Dionysius Exiguus is the source of controversy over determining the beginning of the decade – (Image:

Western countries now use the term AD to refer to the years since Christ’s birth (eg 1,800 AD), and BC (Before Christ) to refer to the year before Christ’s birth, for example 500 BC, or 500 BCE (e.g. 500 BC). Before Common Era), i.e. 500 years before the first year of the 1st century.

Another name that has the same meaning as AD is CE (Common Era/Current Era) to refer to the time from the first year of the 1st century to the present.

After the advent of the Julian calendar, astronomers and scientists of the time knew that the calendar had errors in excess time, but no one had the power to correct the calendar. In 1582, Pope Gregory XIII (1502 – 1585) wanted to correct the Julian calendar so that the Catholic community would celebrate Easter on the correct date, because from 45 B.C. to 1582, real time 10 days more than scheduled.

The pope then issued a decree that only omitted 10 days in October 1582. The time to apply is on October 4, 1582, the next day that should be October 5 must be changed to October 15, ie 10 days disappear overnight and October this year only 21 days.

This second revised Julian calendar is called the Gregorian calendar after Pope Gregory and is popular to this day, but in fact the creator of this new calendar was the outstanding Italian astronomer Luigi Lilio (1510 – 1576).

Currently, according to popular calculation practice , it is calculated that from January 1, 2010 to December 31, 2019 is exactly 10 years, so the start date of the new decade will be January 1, 2020. As for the scientific and astronomical world , the date of the beginning of the first decade of the 21st century is January 1, 2001 and the end of the decade is December 31, 2010, and the second decade is from January 1. 2011 and ends on December 31, 2020, and the third decade we are welcoming will begin on January 1, 2021.

The debate between the two ways of timing will never end because no one will listen. Since governments around the world have not issued regulations on the beginning of a decade, depending on the country, determining the start date of this third decade of the 21st century depends on the customs of the country. they . But for scientists and astronomers, it must be January 1, 2021.

New decade from January 1, 2020 or January 1, 2021?
An ancient Swedish Gregorian calendar of 1753 – (Image:

New decade from January 1, 2020 or January 1, 2021?
Italian astronomer Luigi Lilio made a great contribution to editing the Julian calendar to become the Gregorian calendar -(Photo:Wikipedia)