Ask Dr. Math

Elementary Archive

Browse Elementary Calendars/Dates/Time
Stars indicate particularly interesting answers or good places to begin browsing.

Selected answers to common questions:
    A million/billion seconds.
    Time zones.

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How to Tell Time [11/23/1998]

How do you read time that is not exactly on the hour or half hour? For example, what does 1:35 look like?

One Million Seconds [09/10/2001]

What is a million seconds in weeks, days, hours, minutes, and seconds? What does unit conversion have to do with number bases?

7-, 9-Sided Polygons [9/13/1996]

What is the name of a 7-sided figure? a 9-sided figure? Is one of these ever called a septagon? If "septi" means seven, why is September the 9th month of the year?

Adding and Subtracting Units of Time [12/11/1998]

How do you subtract units of time? In particular, how do you borrow from hours to make the minutes bigger?

Adding Hours and Minutes [01/11/1997]

What time is it 2 hours and 55 minutes after 8:50 A.M.?

Adding Time [04/26/1999]

A spacecraft is launched at 7:45 a.m. on Monday and flies for 5 days, 3 hours, and 18 minutes. What time does it land?

Alternate Method for Adding Time [02/09/2001]

I have come up with a simpler technique for adding and subtracting time values given in hours and minutes, or minutes and seconds. Can you tell me how I can publish my method, which I call the Siri Factor?

Calculating Timecards [08/03/1999]

Is there a formula to find how many hours a person worked?

Converting Clock Time to a Decimal [06/04/1999]

How can you convert a time range to a decimal?

Could There Be 8 Days in a Week? [04/24/2008]

If today is Tuesday, and I'm looking forward to next Tuesday, is it 7 days away or 8 days away?

Counting a Span Versus Finding a Difference between Two Numbers [06/03/2004]

Why is it that the difference between year 2012 and year 2003 equals 10 years but the difference between the numbers 2012 and 2003 equals 9?

Counting Friday the 13ths or Other Dates in a Year [12/17/2003]

What is the greatest possible number of Friday the 13ths that can occur in a year?

Diagramming Days [08/28/2003]

What day of the week was yesterday if 5 days before the day after tomorrow was Wednesday?

Elapsed Time [11/26/2002]

How much time is there between 6:45 and 9:02?

Elapsed Time and the Grouping Factor [03/08/2003]

Why can't you just subtract the two times given in a word problem and get the time elapsed?

How Long is 1000 Seconds? [02/18/2005]

How long does it take to count to one thousand counting one number per second?

Julian Day Calculator [05/18/2003]

If I want to know my exact age in days without counting out the days, what would be the equation?

Midnight on Which Day? [04/12/2003]

I assumed midnight on Sunday meant Sunday at 12:00 AM. So three and half days later would be Wednesday at 12:00 PM or noon...but I was told it was Thursday at noon...

Military and Nautical Time [06/26/2000]

How do you convert nautical (military) time to standard time?

Military Time, Spoken [10/18/2002]

I need to know the correct way of saying the 24-hour clock system.

Numbers: Cardinal, Ordinal, Nominal? [10/25/1999]

Which group of numbers - cardinal, ordinal or nominal - does "time" fit into? Are there other types of numbers as well?

Seconds since 1980 [08/08/2002]

How can I work out the number of seconds since 1980, taking leap years into account?

Subtracting Time [02/27/2001]

A marathon runner crossed the finish line after running for 2 hours, 48 minutes, and 12 seconds. She crossed the line at 9:18:12 A.M. When did the marathon start?

Subtracting Time Expressions [10/09/2005]

How do I subtract two amounts of time, like

  5 years, 21 days, 4 hours, 32 minutes, 17 seconds
- 2 years, 93 days, 7 hours, 47 minutes, 24 seconds
  -------------------------------------------------

Telling Time: A.M. and P.M. [10/30/2001]

What do A.M. and P.M. mean?

Time Zones [05/09/2003]

How do time zones work?

What Day of the Week Was a Given Date? [01/05/2006]

Is there a simple way to figure out what day of the week any date in history fell on?

When Does Your Birthday Fall on the Day You Were Born? [02/20/2004]

I'm curious how many years pass before your birthday falls on the day of the week you were born?

Why Does 1 Degree Equal 60 Minutes? [07/28/2004]

In 60 minutes, the minute hand of a clock makes a full circle, which is 360 degrees. How can 60 minutes equal 1 degree when measuring angles?

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