Lesson Date and Time

DATES AND TIMES

Why Learn About Dates and Times?
As you learn about math, you will learn about basic skills that include arithmetic and geometry. You will also learn about ways to apply those skills to everyday life. This section covers ideas about dates and times. You will also find other sections on our site about units of measurement and money. These concepts are all a part of your life. Your life becomes easier when you understand the concept of passing time.
Never too Young to Date
We're not talking about going out at night for a nice dinner. We think you're ready to know what real dates are. Understanding calendars, years, months, and days is a super-important skill. Even before written calendars, farmers knew the basic seasons of the year. They knew when to plant seeds and harvest crops. You might not need to know about the month to plant your crops, but you probably want to know when you go on vacation, the number of days until a holiday, or when your birthday is. You need to be on the ball and know when those days are coming. That's why you need to understand dates.
Time and Time Again
Every morning you wake up you need to know the time. Every night you probably have parents telling you when to go to bed. Its' also nice to know when your favorite show is on television. To know the answers to those questions, you need to understand time. The easy thing you need to know is how to read a clock. There are also more detailed ideas about time that include how seconds relate to minutes, hours, and days. If you can count, you can tell time.


Calendar Origins
Calendars have been around since the beginning of recorded time. That makes sense since calendars are the things that are used to record the passing of days. While the length and structure of calendars change from culture to culture, they all have one basic unit... a day.
The first calendars were based on the phases of the Moon, but the people who developed calendars quickly switched calendars around to be based on the appearance of stars. They didn't immediately understand it was the amount of time it takes for the Earth to revolve around the Sun. Ancient Egyptians saw that certain stars appeared every 365 days. Using that idea, they determined that was one official cycle. That cycle became known as a year.
Different Systems of Measurement
While the Egyptians were advanced in their measurement of time, many other cultures have created calendars. Prehistoric societies were measuring the passage of time with structures that include Stonehenge and other monoliths. Babylonians set up a measurement system based on the phases of the moon with months about 28 days long. They wound up with a calendar that had 354 days in the year.
The Persian calendar is based on observations as opposed to detailed measurements. They still use the Sun to measure years, but its appearance is measure at the vernal equinox (start of spring). Don't think that calendars were limited to the Mediterranean area. The Mayans and other civilizations had developed their own calendars based on the solar and lunar years. The Chinese also developed their own calendar that was based on a solar year, but contained some elements of the lunar year. You may also hear about Chinese years that are associated with animals.
Our Calendar
We use a calendar that is called the Gregorian calendar. The calendar has it's roots in the 300s, but was not refined and used by western nations until 1582. Our modern calendar breaks the year into 365 days and twelve months. Each month has a different number of days ranging from 28 to 31. Every four years, the second month (February) has 29 days to adjust for flaws in the calendar. That special year is called a leap year.
While we measure years with months and days, major solar events still occur at the same time each year. The year starts in winter and the vernal equinox (start of spring) happens on about March 21. The summer solstice (start of summer) happens on about June 21. Fall begins at the autumnal equinox around September 23. Finally, winter begins on about December 21 with the winter solstice. We use the word "around" because the exact exact time that the moments occur changes every year.
Details to Remember
- Our calendar is based on a solar year.
- There are 365 days in a normal year and 366 days in a leap year.
- There are 12 months in a year.
- There are four seasons in a year.
What is a Year?
You can pick up a calendar and see that a year is something that lasts from January 1 to December 31. After that, It starts over again. What is a year beyond dates on a piece of paper? A year is the time it takes the Earth to orbit the Sun one time. That time is equal to 365.24 days. You should know that it changes a little each year because when you travel 574,380,400 miles (924,375,700 km) sometimes you go a little faster or slower. Stuff happens along the way.
As we move around the Sun, days change and four seasons begin to appear. Seasons exist because the Earth does not sit up and down. It's on a bit of a tilt. That tilt allows the day lengths to change throughout the year. Seasons with longer and warmer days are summers and the shorter and colder days are winters. The differences in the seasons are not as noticeable on the equator. As you move towards the poles, they become more and more extreme. Summers at the poles are almost entirely daylight and the winters are almost entirely night.

Leaping Like a Frog
Now you know a year is a little more than 365 days. A day is a little less than 24 hours long. All of this "little less" and "little more" adds up through the years. That leads s to the idea of leap years. Every four years we need to adjust our calendars. We add a special little day in February. Normally there are 28 days in February, but leap years give it 29 days. That extra full day makes up for the extra quarter days we gain every four years.

Year Terms
A year is one unit. It's one trip around the Sun. We'll talk about months and days in the next sections. Let's look at some terms used to describe multiple years.
Decade: Ten (10) years.
Century: One hundred (100) years.
Millennium: One thousand (1,000) years.
Light Year: The distance light can travel in one Earth year. (Almost 6 trillion miles)
There are also terms used to describe millions of years. These aren't exact numbers like a decade or century. These terms describe general geologic periods.
Eon: The longest amount of time.
Era: Several eras make up an eon.
Period: Several periods make up an era.
Epoch: Several epochs make up a period.
So where are we now?
Phanerozoic Eon - Cenozoic Era - Quaternary Period - Holocene Epoch - This Year - Today
We just thought you might like to know. It's like addressing an envelope.
Details to Remember
- There are 365 days in a normal year and 366 days in a leap year.
- Four Seasons in a year (Winter, Spring, Summer, and Fall)
- A decade has ten (10) years.
- A century has one hundred (100) years.
- A millennium has one thousand (1,000) years.
Breaking Down a Year
Even though we base a year on a 365 day value, it's a little hard to describe a year in quantities of 365. Let's say that someone came to you and said "What are you doing on the 185th day of the year?" It might be a little hard to answer. To make it easier to understand different times of the year, we have months. There are twelve (12) action-packed months every year. Here they are:
January (Month 1)
February (Month 2)
March (Month 3)
April (Month 4)
May (Month 5)
June (Month 6)
July (Month 7)
August (Month 8)
September (Month 9)
October (Month 10)
November (Month 11)
December (Month 12)
So now someone can come up and say, "What are you doing on July fourth?" Then you'll know how to answer and show up for the barbeque on time. A side note for you to remember is that there are different numbers of days in each month. We have a list at the bottom of the page.
A Little About the Moon
So how did we come up with twelve months in a year? It has to do with the phases of the Moon. The Moon goes through a full phase (from new moon to new moon) every 27.3 days. That 28ish day period is roughly as long as a month. As we said, that's how it all started. Over the years, the lengths of months were redefined into the odd collection of days we have now. Here are the phases of the Moon and the number of days each phase appears after a new moon. The days are approximate.
New Moon: Day 0 or Day 28ish (The moon is dark.)
Waxing Crescent: Days 3-4 (There is only a sliver of a moon visible.)
First Quarter: Day 7 (Half of the moon is bright.)
Waxing Gibbous: Day 10-11
Full Moon: Day 14 (The entire moon is lit.)
Waning Gibbous: Day 17-18
Last Quarter: Day 21 (Half of the moon is bright.)
Waning Crescent: Day 24-25 (There is only a sliver of a moon visible.)
New Moon: Day 28ish or Day 0 (The moon is dark.)
Details to Remember
Months of the year:
- January (31 Days)
- February (28 Days, 29 on Leap Year)
- March (31 Days)
- April (30 Days)
- May (31 Days)
- June (30 Days)
- July (31 Days)
- August (31 Days)
- September (30 Days)
- October (31 Days)
- November (30 Days)
- December (31 Days)
Weeks and Weekends
We've given you scientific and astronomical reasoning for the length of years and days. Do you think we have the same background for weeks? No. Weeks are a bit made up for our society. They are a cultural development. They help when you need to schedule things or to plan, but there is little science involved in the length of a week.
There are many origins to a week. Some evidence points to cultures that gathered for market days. There is also a lot of historical evidence that points to religious origins for a week. People used to work for six days and then have one day off to worship. Even today, you will find many religions that choose a day for religious purposes. Depending on the religion, it could be Friday, Saturday, or Sunday. In the modern world, Monday through Friday are work days and Saturday and Sunday are classified as weekends. Office workers do not usually work on the weekends. Speaking of days… What are the days of the week?
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
Weeks and Months
We said there was a little science involved in the length of a week. That little bit is a good coincidence between the length of four weeks and the cycle of the Moon. The Moon goes through a complete cycle (new moon to new moon) about every twenty-eight (28) days. There have been calendars based on 28 day months and there is a good chance that four weeks is about the same length of time. Don't look for many connections between weeks and months in our calendar.

Days and Weeks
You've got months, weeks, and now days. There are seven days in each week. What is a day? Now you can pick up a science book. A day is the time it takes the Earth to make one complete spin or rotation. The Earth rotates around it's axis about once every twenty-four (23.93) hours. Four our convenience, clocks measure days in two blocks of twelve hours. The first twelve hours start at midnight and are labeled as AM. hours. AM. stands for ante meridiem, which is Latin for "before noon." The next twelve hours are labeled PM. PM. stands for post meridiem, which is Latin for "after noon."
There are different ways of displaying time that you might hear about. The military uses a different system based on a 24-hour clock. While you might talk about 4 PM for the start of your favorite cartoon, a military man might say, "The cartoon begins at sixteen hundred hours." They count from zero to twenty-four. It's the same for the AM hours, but the PM hours require you to add twelve to the number you see on the clock.
Military Conversion:
Thirteen hundred hours = 1:00 PM
Fourteen hundred hours = 2:00 PM
Fifteen hundred hours = 3:00 PM
Sixteen hundred hours = 4:00 PM
Seventeen hundred hours = 5:00 PM
Eighteen hundred hours = 6:00 PM
Nineteen hundred hours = 7:00 PM
Twenty hundred hours = 8:00 PM
Twenty-one hundred hours = 9:00 PM
Twenty-two hundred hours = 10:00 PM
Twenty-three hundred hours = 11:00 PM
Details to Remember
- There are seven days in a week
- There are twenty-four hours in one day.
- Monday (Day 1)
- Tuesday (Day 2)
- Wednesday (Day 3)
- Thursday (Day 4)
- Friday (Day 5)
- Saturday (Day 6)
- Sunday (Day 7)
Breaking up a Day into Hours
A day is made up of twenty-four (24) hours. What is an hour? As far as your ability to tell time is concerned, an hour is made up of sixty (60) minutes. The twenty-four hours in every day are broken up into two sets of twelve hours. The first set of twelve is the AM period and the second set is the PM period. AM is from midnight to noon while PM is from noon to midnight.
Why do we use twelve hour amounts? It has to do with an old counting system that was based on the number twelve. We wound up with twelve months, hours, horoscope signs, and the number of donuts you might buy (a dozen). Although times may have changed and we now move towards metric systems of measurement, the twelves of the world will always be with us.
Hours Used in Measurement
Hours are great for business. People go to work for eight hours a day. They go to lunch for one hour a day. They might even have a commute for one hour a day. Hours divide our days into bite-sized pieces that we all understand. You have probably been told you need eight hours of sleep or that you can only watch two hours of television each day. Hours are everywhere. They may our daily schedules possible.
Hours are also great when you need to round off times. Let's say your drive to work takes you fifty-five (55) minutes. You don't tell people fifty-five minutes. You round up to an hour because it's easier to understand and shorter to say. If you worked for eight hours and twelve minutes, you would probably round off the value to eight hours. People don't have to think when they hear eight hours. For casual conversation, eight hours and twelve minutes is too detailed.
Common Amounts of Time:
Time at the gym each day: 1 Hour
Length of a movie: 2 Hours
Nightly prime-time television: 3 Hours
Time difference between Los Angeles and New York: 3 Hours
Time your food spends in the small intestine: 4 Hours
Time for sleep each night: 8 Hours
Length average work day: 8 Hours
Length of a day: 24 Hours
Lifespan of Gastrotrich (shortest lifespan): 36 Hours
Length of the weekend: 48 Hours
Details to Remember
- There are 24 hours in a day.
- There are about 12 hours of light and dark each day.
- There are 60 minutes in each hour.
- A clock shows time for 12 hours and then repeats.
MINUTES
Breaking an Hour Into Minutes
You know that there are twenty-four (24) hours in a day. Twelve of those hours are in the a.m. and twelve are in the p.m. It's nice to know about hours, but the world needed a smaller division of time. We needed minutes. There are sixty (60) minutes in each hour. The value of sixty has its origins in the Babylonian counting system. They had a system that worked with a base of sixty. If we created time today, there is a chance we would have one hundred (100) smaller minutes for each hour.
When you look at the face of a clock, you will see the major division for the hours. There are also smaller notches that represent the minutes. You will find four notches between each hour division. There are also quick ways of telling someone the number of minutes. You could say "Four forty-five" or the faster way… "A quarter to five." Here are some of the abbreviations:
Quarter After: Fifteen minutes past the hour. (Quarter after three = 3:15)
Half Past: Thirty minutes past the hour. (Half past four = 4:30)
Quarter To: Fifteen minutes before the next hour. (Quarter to eight = 7:45)
Minutes Used Every Day
The standard scientific unit for measuring time is the second. Minutes are usually used to measure slower events. You will probably use minutes to measure the length of your television shows. There are thirty (30) minute, sixty (60) minute, and two-hour (120 minute) shows on every night. The next place you will use minutes is in school. Every school has a different schedule, but your days are probably broken down to the very minute with class time, lunch, and breaks. Knowing how to count minutes is very important when you have a schedule. After home and school, your parents are very familiar with minutes at work. They may measure tasks and projects down to the minute. One day they might spend thirty-five minutes on one project and fifteen on another. They need to keep daily records of their time and they always use minutes.
Details to Remember
- There are 60 seconds in each minute.
- There are 60 minutes in each hour.
SECONDS
You need to know that there are sixty (60) seconds in each minute. Why do you need to know about seconds? Clocks work on a system of seconds. They count down sixty movements of a second hand to complete a minute. But why sixty? In the same way that there are sixty minutes in an hour, a minute was originally broken up into sixty seconds because that's how it was done for hundreds of years. It all started with the Babylonians who liked to measure things with amounts of sixty. If clock systems were rebuilt today, we might break hours down into hundred minute increments and those minutes might have one hundred seconds each. You never know.
Seconds Used in Science
Seconds are a fundamental unit in science. They are classified as the SI unit for time. SI Units are an international system of units that all of science uses. It helps to make observation and measurement consistent no matter what your experiment or your type of equipment. Because so much in science happens in a short amount of time, seconds have been used as the fundamental unit. In physics you will learn about meters per second. You may read about liters per seconds that would be a measure of the flow of a fluid. If you see a "per second" in the units you will be looking at the speed of something.
Smaller Than a Second
Is there anything smaller than a second is? Sure there is. For units of time smaller than a second you need to know that we use the decimal system, not units of sixty. You see amounts smaller than a second in all sorts of sports and science activities. How fast does someone run? How much time is left on the clock? How long did it take for that reaction to occur. It all needs to be measured in amounts smaller than a second. Here are some examples:
14.5 Seconds = Fourteen and five tenths of a second. (Basketball)
14.58 Seconds = Fourteen and fifty-eight hundredths of a second. (Track and Field)
14.583 Seconds = Fourteen and five hundred eighty-three thousandths of a second. (Chemistry)
Details to Remember
- There are 60 seconds in each minute
- Amounts smaller than a second are measured with decimals.