Short answer: The 21st century and the third millennium began on January 1, 2001. That’s because our calendar counts years starting at 1, not 0, making 2000 the final year of the 20th century. Still, the world partied in 2000 because digit rollovers are irresistible milestones and easy to market—and that’s a different kind of logic.

2000 or 2001? The quick explanation

Centuries and millennia are ordinal blocks of 100 and 1,000 years respectively. In the Gregorian calendar (the civil calendar used worldwide), the first century runs from year 1 through 100, the second from 101 through 200, and so on. By that rule, the 20th century is 1901–2000, and the 21st is 2001–2100. The third millennium is 2001–3000.

Why 2001 is the official start

Ordinal counting vs. digit rollover

When people ask “2000 or 2001?”, they’re comparing two different counting systems:

• Ordinal counting (official): 1st through 100th year = first century; 101st through 200th = second century; and so on.
• Digit rollover (popular): Years with all zeros feel like clean starts—1900, 2000, 2100—even though ordinals say they’re the last years of a century.

No year zero in the Gregorian calendar

The Anno Domini/Common Era system, introduced in the 6th century by Dionysius Exiguus, begins at year 1. There is no “year 0” in the civil Gregorian or Julian calendars. Because of that, the first block of 100 years ends at 100, not 99; the first block of 1,000 ends at 1000, not 999. The pattern repeats every century and millennium.

Calendar reforms didn’t change year numbering

The Gregorian reform in 1582 adjusted leap-year rules and skipped 10 dates to realign the calendar with the seasons, but it did not insert a year zero or otherwise alter the ordinal structure. Year numbering stayed continuous, preserving the 1–100, 101–200 sequence.

What official sources say

Timekeeping authorities and reference institutions have consistently stated that the 21st century and third millennium began on January 1, 2001. The U.S. Naval Observatory and the Royal Observatory Greenwich, among others, have published explanations pointing to the absence of a year zero and the ordinal nature of centuries and millennia.

So why did we celebrate in 2000?

“Millennium Fever” peaked on the night of December 31, 1999, because of human psychology and media-ready moments, not calendar arithmetic. Three forces drove the 2000 celebration:

• Digit drama: All four digits flipped at once. That’s rare and visually satisfying in a way “2001” isn’t.
• Marketing momentum: From “Millennium” concerts to “Y2K” television specials, 2000 was easier to brand. Major projects—from bridges to museums—timed their unveilings for January 1, 2000.
• Y2K headlines: The “millennium bug” made 1999→2000 a technical and cultural story, amplifying the moment regardless of historical purism.

Examples abound: the Millennium Dome opened in London on January 1, 2000; the United Nations hosted a Millennium Summit in 2000; cities from Sydney to New York staged record-breaking fireworks when the digits rolled over. None of this contradicted the official boundary; it simply celebrated the spectacle.

Anniversary math you already use

To see why 2001 is correct, use a familiar analogy—birthdays and anniversaries:

• Birthdays: A baby is born at age 0. The 1st birthday marks the completion of the first full year. Likewise, year 100 closes the first century; year 200 closes the second, and so on.
• Anniversaries: A “100th anniversary” occurs 100 years after an event. If something happened in 1901, its 100th anniversary is in 2001, not 2000.

This is how media “On This Day” features tally milestones and how institutions plan centennials and bicentennials. The math is consistent: the “nth” anniversary happens after completing n whole years.

How calendars count: Gregorian vs. astronomical and ISO

Gregorian (civil) calendar

The Gregorian calendar—the basis for everyday civil life—has no year zero. It uses ordinal centuries and millennia as described above. In this system, 2000 is the last year of the 20th century, and 2001 kicks off the 21st.

Astronomical year numbering and ISO 8601

There is one wrinkle, mostly of interest to technical readers. Astronomers (and the ISO 8601 date format used in data exchange) use a proleptic system that includes a year zero for arithmetic convenience:

• Astronomical/ISO year 0000 corresponds to 1 BCE.
• Astronomical/ISO year −0001 corresponds to 2 BCE.

This helps with calculations spanning BCE/CE, but it is not how civil or historical centuries are defined. When people ask “2000 or 2001?”, they are asking about civil usage. By that standard, the 21st century begins in 2001.

Common confusions cleared up

• Is 2000 in the 20th or 21st century? 2000 is in the 20th century; 2001 is the first year of the 21st.
• Do decades work the same way? Not always in everyday speech. “The 1990s” means 1990–1999 (digit-based). But the “201st decade” in strict ordinal terms would be 2001–2010. Media usage usually favors digit names for decades, but centuries and millennia stick to ordinals.
• Is 1900 a century year? Yes; it is the final year of the 19th century (1801–1900). The 20th century begins in 1901.

How institutions date major commemorations

Museums, archives, and governments tend to follow ordinal rules when marking historical anniversaries, because they’re tied to specific dates:

• Event in 1900: Centenary in 2000.
• Event in 1901: Centenary in 2001.
• Event in 2000: 25th in 2025, 50th in 2050, 100th in 2100.
• Event in 2001: 25th in 2026, 50th in 2051, 100th in 2101.

This is why you’ll see “On This Day” entries and centennial exhibits line up precisely with event dates, not rounded millennial pageantry. For instance, the centenary of Queen Victoria’s death (1901) fell in 2001; the Wright brothers’ first flight (1903) was marked in 2003; the Titanic centenary (1912) occurred in 2012. The pattern is steady and uncontroversial.

Media and "On This Day" entries

Calendar-driven media segments often bridge both worlds—celebrating digit moments while adhering to ordinal anniversaries. You’ll commonly see entries such as:

• “Dec. 31, 1999: The world welcomes the year 2000.”
• “Jan. 1, 2000: Last year of the 20th century begins.”
• “Jan. 1, 2001: The 21st century and third millennium officially begin.”

This strikes a balance between the cultural spectacle of 2000 and the calendrical rule that makes 2001 the official start.

The deeper logic: ordinals, cardinals, and names

At the heart of the 2000-or-2001 debate is a simple linguistic point:

• Ordinal terms (first, second, twenty-first) refer to position in a sequence and include the endpoint. The “21st century” is the 21st block of 100 years since year 1.
• Cardinal terms (1, 2, 21) count quantities. We like cardinals when we see “2000” because it looks like a big round number. But the name “21st century” is ordinal, so its boundary follows ordinal rules.

Practical guide: what to write and say

• “The 21st century began on January 1, 2001.”
• “The year 2000 was the final year of the 20th century.”
• “The third millennium of the Common Era runs from 2001 through 3000.”
• “Most people celebrated in 2000 because of the digit rollover, not because of the official boundary.”
• “Anniversaries are counted inclusively: the 100th anniversary occurs 100 years after an event’s date.”

Why this matters (beyond trivia)

Understanding century boundaries isn’t just pedantry. It helps you:

• Fact-check timelines: Assign historical events to the correct century.
• Plan commemorations: Time centennials and bicentennials to the exact year.
• Write accurately: Avoid common misstatements in reports, exhibits, and "On This Day" features.

Bottom line

By the arithmetic of the calendar we use every day, the 21st century and the third millennium began on January 1, 2001. The massive celebrations in 2000 were about the thrill of seeing the digits change—an understandable and memorable choice, but not the formal boundary. If you’re dating a commemoration, writing a headline, or building an exhibit, follow ordinal rules. If you’re throwing a party, follow your heart—and the fireworks schedule.

FAQ

Did the 21st century start in 2000 or 2001?

Officially, 2001. The civil calendar has no year zero, so each century runs from xx01 to xx00. That makes 2000 the last year of the 20th century and 2001 the first year of the 21st.

Why do some sources say 2000?

Because “2000” is a round-number milestone that feels like a fresh start. Many organizations chose to celebrate then for cultural and marketing reasons. But in formal terms, the new century began in 2001.

Is there ever a year zero?

Not in the civil Gregorian or Julian calendars. Astronomers and the ISO 8601 data format use a year zero for calculations (with 0000 representing 1 BCE), but that convention doesn’t define historical centuries.

Which century is the year 2000 in?

The 20th century. The 21st century begins on January 1, 2001, and runs through December 31, 2100.

Do decades work the same way as centuries?

Everyday language treats decades by their digits (“the 1990s” = 1990–1999). But when people speak of the “first decade” or “201st decade” in strict ordinal terms, those blocks would run 1–10, 2001–2010, etc. Media usage favors digit names for decades and ordinal names for centuries.

How should institutions schedule centennials and bicentennials?

Use the event date itself: a 100th anniversary occurs exactly 100 years later. An event in 1901 has its centenary in 2001. This is how museums, archives, and “On This Day” features date commemorations.

What about the third millennium—when does it end?

The third millennium runs from 2001 through 3000, inclusive. The fourth millennium begins on January 1, 3001.