“Decade”, “Century”, “Millennium”… – Here’s What Comes Next

Marcus Froland

Are you curious about the timeline of history? Have you ever wondered what comes after a millennium?

In this article, we’ll explore the different eras of time and answer the question: What comes after a millennium? We’ll look at decades, centuries, millennia, and eons.

By the end, you’ll have a better understanding of how these units fit together in our world’s timeline.

Key Takeaways

  • Decades, centuries, and millennia are shorter units of time used for measuring the passage of time and marking important events in history.
  • Earth’s geological time is divided into eons, such as the Phanerozoic and Proterozoic, which are further subdivided into eras, periods, epochs, and ages.
  • Larger units of time include supereons, eras, periods, and epochs, each spanning different durations of time.
  • Decades and millennia can also be understood from cultural and religious perspectives, with associations to specific values, trends, and designated calendar years.

What Is a Decade

A decade is a period of ten years. It can be used to measure the passage of time or to mark important events in history.

A decade is divided into two five-year periods known as half decades, and each year can be further divided into months, weeks, days, hours, minutes, and seconds.

Decades are often associated with a particular set of cultural values and trends that become prevalent over a ten-year period. For example, the 1920s were known for flappers and jazz music while the 1950s were characterized by rock ‘n’ roll and Elvis Presley.

The terms ‘decade’, ‘century’, and ‘millennium’ are all based on units of 10 years but represent increasingly longer spans of time.

What Is a Century

You’re living in a century, which is comprised of ten decades. A century is a period of 100 years, and it’s the largest unit of time used to measure historical events. It’s also the longest division of time that uses numbers instead of names.

For example, the twenty-first century started on January 1st, 2001 and will end on December 31st, 2100. Consequently, centuries are commonly referred to by their starting date (e.g., 21st century) or ending date (e.g., 20th century).

In addition to being used for measuring historical time periods, centuries can be divided into smaller units like decades and years for more specific measurements.

What Is a Millennium

A millennium is a period of time that spans 1,000 years. It is divided into centuries and decades, which are smaller portions of the larger span.

A millennium is most often used to refer to specific periods in history, such as the start and end of an era or a major event. Here are four ways we can talk about a millennium:

  1. Calendar Year – We can designate the start and end dates of each millennium by its calendar year; for example, the 20th century started in 1901 and ended in 2000.

  2. Timeline – We can also use a timeline to measure how long ago something occurred within a given millennium; for example, the early 17th century would be 1601-1700 during the 17th century and 1101-1200 during the 12th century.

  3. Culture – We can look at a given period through its culture; for example, some consider 1993-2003 to be ‘the Millennium’ because it marked popular music trends like boy bands and hip hop becoming mainstream genres.

  4. Religion – Lastly, we can explore religious aspects of any given period; for example, many people associate 1000 AD with Christianity when it spread throughout Europe following Charlemagne’s reign in 800 AD.

What Is a Eon

An eon is a very long period of time that is longer than a millennium, typically spanning over one billion years. It is the largest unit of time in geology and used to describe major changes in Earth’s history.

Geologists have divided up Earth’s history into two separate eons: the Phanerozoic Eon, which started 541 million years ago and continues today, and the Proterozoic Eon which lasted from 2.5 billion to 541 million years ago.

Each eon consists of many eras, periods, epochs, and ages that help scientists understand how Earth has evolved throughout its 4.6-billion-year history.

The current era we are living in is known as the Holocene Epoch and it began about 11,700 years ago at the end of the Pleistocene Period.

Beyond the Eon – What Comes Next?

Beyond the eon, there’re even larger units of time that help scientists understand how Earth has evolved.

The next largest unit is a supereon, which spans 200 million years or more.

After that comes an era, lasting between one and two hundred million years.

Then comes a period, typically lasting 10-30 million years.

Lastly, there’s an epoch—a shorter period of time lasting one to three million years.

All these larger units provide context and help us grasp just how ancient our planet really is!


You now know what a decade, century, millennium, and eon are.

But beyond the eon is another concept: an ‘aeon’. An aeon is a much longer period of time that can last billions of years.

It’s interesting to think about how far we’ve come in such a short span of time, and it’s remarkable to consider the future possibilities as we continue our journey through the age of the universe.