11 Common Units Of Time and Evolution of Calendar, Clocks

Explore the fascinating world of various units of time and their durations, from the tiny fraction smaller than a second to an interval that lasts millions of years.

Time is the measure of duration and change of events, using units such as seconds, minutes, hours, days, years, etc. Yet, these are just a few examples of a bunch of time measurement units available. Interestingly, people throughout history used diverse tools and devices to track and measure time.

See more: Concepts of Time from Philosophical & Scientific Perspectives


Calendars and clocks, whose history stretches back thousands of years BCE, are among the most enduring. Now, join us as we delve into the fascinating world of units of time and explore the evolution of clocks and calendars. Keep scrolling to learn more!

What is a Unit of Time and SI Unit of Time?

To define, units of time are standardized quantities of time. People use them to measure or indicate the length of time, the interval of an event, or the duration between two events.

In other words, the length or frequency of something that happens or lasts can be measured or expressed by using a unit of time. Units of time provide a framework for organizing and measuring the passage of time on various scales, from fractions of a second to millennia or even petaseconds, and more.

Different cultures may use specific calendars or systems to track time, and some units may have variations in their exact lengths based on context and convention.

Use units of time to measure or indicate the length of time, the interval of an event, or the duration between two events.

Use units of time to measure or indicate the length of time, the interval of an event, or the duration between two events.

The second (s or sec) is often referred to as a base time unit in the International System of Units, or the SI unit of time. The second, as per the SI, is described as a time interval equivalent to “9,192,631,770 periods of the radiation corresponding to the transition between the 02 hyperfine levels of the cesium-133 atom’s ground state”.

As the base time unit, the second serves as the basis for many other time units used in everyday life and scientific measurements.

A List Of The 11 Most Common Units Of Time

Terms like seconds, minutes, hours, and years have been familiar to any of us since we were all children. Yes, they are among the most fundamental time units. For centuries, people around the world have used hours, days, years, and a bunch of other time units to measure and calculate time and time intervals.

However, there are many other units of time, each with a different length. Here’s a look at the 11 most common time units and how long they are in seconds—the base unit of time.

Common Units of Time

Millisecond (ms)

1000 milliseconds = 01 second

Second (s)

Base Unit

Minute (min)

01 Minute = 60 seconds

Hour (Hr)

01 Hour = 60 minutes or 3,600 seconds


01 Day = 24 hours, or 1440 minutes, or 86,400 seconds

Week (wk)

01 Week = 7 days, or 168 hours, or 10,080 minutes, or 604,800 seconds


01 Month has from 28 to 31 days, or from 2,419,200 to 2,678,400 seconds.

01 Month ~ 4 weeks.


01 Year has 365 or 366 days* ~ 31,536,000 or 31,622,400 seconds.

01 Year ~ 52 weeks.


01 Decade = 10 Years


01 Century = 100 Years


01 Millennium = 1000 Years

 Note: 1 year typically equals 365 days, but a leap year has 366 days, one day more than usual.

Among these 11 time measurement units, the millisecond is the smallest unit of time, while the millennium is the biggest. Interestingly, there are various time measurement units that are smaller than a millisecond and some even longer than a millennium. Of course, they are not used commonly in reality. Here are some examples:

Unit of Time

Equal to seconds


1,000,000 microseconds = 1 second


1,000,000,000 nanoseconds = 1 second


1,000,000,000,000 picoseconds = 1 second


01 Terasecond ~ 31,709 years


01 Megaannum = 1,000 millennia or 1 million years


01 Petasecond ~ 31,709,791 years.

Inevitable Tools To Measure And Organize Time - Calendar and Clocks

Without Calendar and Clocks, we would not have been able to measure the passage of time and keep track of time in different units and formats. Calendars help us understand the structure of days, weeks, months, and years, while clocks provide a more immediate and specific measurement of time throughout the 24 hours of the day.

Calendars show the organization of days, weeks, months, and years, while clocks offer a precise measure of time within the 24-hour day.

How Calendars Shape Our Lives

A calendar, to be more specific, is a system of organizing days, weeks, and months based on a specific natural or artificial cycle, such as the solar cycle (Solar Calendar), phases of the moon (Lunar Calendar), the seasons of the earth, or even a religious or cultural event.

Calendars are essential for tracking days, weeks, months, and years, and they are used for various purposes, including scheduling events, agricultural planning, religious observances, etc.

As of today, the solar calendar based on the Gregorian calendar is known as one of the most popular tools of timekeeping. Briefly, there are 365 days (or 366 days in leap years) divided into 12 months with different lengths in the solar calendar, with February being the shortest month with 28 or 29 days.

What are Clocks and Time?

Clocks, on the other hand, are devices or tools that people use to measure and display time and the passage of time, designed to indicate seconds, minutes, and hours of the day. They can be found in many shapes and sizes, from simple sundials to complex atomic clocks, of which mechanical, digital, or electronic are among the most popular.

Clocks can be used for various purposes, such as telling the time, setting alarms, coordinating events, etc. Clocks are also symbols of human creativity, ingenuity, and curiosity.

Delve Into History of Time and Calendars

The history of calendars, which is deeply rooted in the human need to understand the passage of time and keep track of it, dates back thousands of years ago. Early calendars were created based on astronomical observations and natural phenomena like the movements of the stars, moon cycles, annual flooding of the river, etc.

Some of the oldest calendars were found in ancient civilizations, such as the Sumerian, Egyptian, Babylonian, Persian, Hebrew, and Maya.

Later, calendars were developed by different cultures and regions, such as the Chinese, Indian, Islamic, Roman, and Gregorian calendars. These calendars were more accurate and complex, using mathematical and astronomical calculations to adjust the length and structure of the year.

Today, most parts of the world use the Gregorian calendar—the calendar introduced by Pope Gregory XIII in the late 16th century to reform the Julian calendar. It’s a solar calendar that has 12 months and 365 or 366 days, depending on whether it is a leap year or not.

Throughout history, different cultures and civilizations have devised their own calendars to align with religious, agricultural, or political needs. Calendars continue to evolve, adapting to the changing needs of society and providing a framework for organizing time.

Units of time play a crucial role in our everyday lives and are fundamental to how we organize, plan, and navigate the world around us. Time measurement units are many and various; they can be as small as a fraction of a second and as large as millions of years.

However, seconds, minutes, hours, days, years, and more are the most common units we use frequently, of which the second is the base unit. Whether measured in brief intervals or extended epochs, each unit carries its own narrative, contributing to the grand tapestry of shared history.