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The Difference Between the Millennium and the Year 2000

Short Explanation

The 3rd Millennium starts on January 1 of the year 2001, NOT on January 1 of the year 2000 as many people who use the Gregorian calendar believe..
Year 2000 starts January 1st, year 2000.

Longer Explanation

The reason why the 3rd Millennium, also known as the 21st Century, starts in 2001 is because there was no year 0, i.e., there was no year AD 0, nor was there a year 0 BC. The year before the year AD 1 is defined as the year 1 BC, so the year 0 was skipped. (See below). Therefore, January 1, year 1, is defined to be the start of the 1st century and the start of the 1st Millennium.

Because one Millennium is 1000 years, the first Millennium ends with year 1000. The next (2nd) Millennium starts 1000 years after the first, that is in year 1+1000 = 1001. And the 3rd one starts 1000 years later than the 2nd: 1001+1000 = 2001. The same procedure could be followed for centuries. See the tables below for more information.

What Should We Celebrate?

We can happily celebrate both years, although probably most people will think the start of year 2000 is a bigger celebration than the start of year 2001.
At the start of the year 2000, we celebrate that it's a special round number, and at the start of the year 2001, we can celebrate a new century, a new Millennium, and that 2000 years have passed since year 1 started.

Why Was There No Year 0?

When the present system we use to count years was invented by a scholar called Dionysius Exiguus in the 6th century, and later established around Europe, Roman numerals were used, which did not have a zero. Therefore 1 BC is the year before AD 1, with no intervening year 0. (Year sequence: 3 BC, 2 BC, 1 BC, AD 1, AD 2, AD 3, etc...).

Exceptions – Other Calendars

There are many other calendars in use other than the Gregorian Calendar, e.g. Jewish, Muslim, Buddhist, that started the numbering of years for the initial epoch at a different time than the Julian/Gregorian calendars, and therefore they do not celebrate the 3rd Millennium at the same time. However, most of them still use the Gregorian Calendar for civil use.

Starting Dates For Centuries

Century-No.First DayLast Day
1st1st of January 131st of December 100
2nd1st of January 10131st of December 200
16th1st of January 150131st of December 1600
17th1st of January 160131st of December 1700
18th1st of January 170131st of December 1800
19th1st of January 180131st of December 1900
20th1st of January 190131st of December 2000
21st.1st of January 200131st of December 2100
22nd1st of January 210131st of December 2200

Starting Dates for Millennia

Millennium-No.First DayLast Day
1st1st of January 131st of December 1000
2nd1st of January 100131st of December 2000
3rd1st of January 200131st of December 3000
4th1st of January 300131st of December 4000

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