See also the
Dr. Math FAQ:
calendar & days
of the week
3D and higher
Browse Middle School Calendars/Dates/Time
Stars indicate particularly interesting answers or
good places to begin browsing.
Selected answers to common questions:
A million/billion seconds.
- Deriving Zeller's Rule [02/25/2003]
How do you derive the formula f=k+[(13*m-1)/5]+d+[d/4]+[c/4]-2*c?
- 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?
- 12-Hour and 24-Hour Time [04/25/2002]
If it is 00:46 on the clock, what would the time be for 12 hour time?
- 24 Hours in a Day [08/15/2001]
Why divide the day into 24 equal parts? Why not 23 or 25?
- 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?
- Analog and Digital Time [11/15/2004]
What is 4:05 in analog time?
- Averaging Minutes and Seconds [07/03/2002]
I need the average time of 20 different times. If I enter, say, 4
minutes and 25 seconds as 4.25, I come up with the wrong average. How
should I do it?
- A Billion Seconds [10/21/1996]
If I was born on 01/02/84, when will I have been alive for a billion
- Birthdays Every Year [05/14/1999]
Why does my birthday day of the week always go up each year?
- Calculating Average Starting Time [06/23/2003]
How do you average a series of starting times to get the average
starting time? Say 9:30a.m., 10:15a.m., 11:45a.m., 1:30p.m., and
- Calculating How Many Days between Two Dates [10/06/2004]
I'm trying to figure out how may days it will be before we reach July
10, 2006, and I can't figure out how to go about it other than taking
a calendar and counting by hand. Is there a way to calculate it?
- Calculating Timecards [08/03/1999]
Is there a formula to find how many hours a person worked?
- Calendar Repeats [05/06/2002]
What is the next year after 1991 with the same calendar as 1990?
- Changing Time Zones [11/17/1999]
If an airplane leaves New York at 09:15 to travel to London, and the
flight takes 5 hrs. 45 min., what will the local time be when the plane
- China's Time Zone [12/01/2000]
Why does the former USSR have 11 time zones, while China has only 1?
- Clock Hands and Hours Worked [01/29/2003]
A man arrives at work between 8:00 and 9:00 a.m. at exactly the
moment the minute hand and hour hand of the clock point in the same
direction.... How many hours did the man work?
- Conversion of Time Range into Minutes [11/13/2002]
What equation/conversion can I use to find the number of minutes
between the start and end times for both 'available' and 'actual' time
on a 24-hour clock?
- Converting Gregorian Date to Gregorian Days [02/27/2003]
I'm trying to convert a Gregorian date (e.g. Feb. 10, 1788) to the
number of days that has passed since the implementation of the
Gregorian Calender on October 15, 1582 (this being day 1).
- Converting HH:MM to Minutes in Excel [06/25/2003]
Is there an easy method (i.e. any formula or a macro) for converting
Hours:Minutes (HH:MM) to Minutes in Excel?
- Converting Time [8/30/1996]
I have a clock that is "measured by 100", meaning 25=15 minutes, 50=30
minutes, etc. How do I convert this into "real time"?
- 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
- 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?
- Dated Determination: Converting from the Julian Calendar to the Gregorian [02/09/2011]
A programming student seeks an algorithm that would convert Julian calendar dates to
Gregorian, and back again. Doctor Vogler acknowledges the flexibility of using a
common dating scheme as a base, then outlines how to leverage the offset between
the two systems.
- Define Warp in Lightyears [3/31/1995]
On Voyager, the new Star Trek show, they are 75 years from home. Their
top speed is Warp 9.75. Can you define "Warp" in lightyears?
- Different Yearly Calendars [09/30/2001]
How many years does it take before each calendar is used at least once?
Start counting with the year 2001.
- 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?
- Formula for Easter [06/12/1999]
Is there a formula for finding the month and the day on which Easter
falls in a given year?
- Formula to Compute One's Exact Age [05/31/2000]
Is there an equation to find a person's exact age on a given date, taking
into account leap years?
- Good Friday on the 13th [04/10/2001]
How many times has Good Friday fallen on the 13th?
- How Long Does It Take to Count to a Billion? [03/28/2003]
Why people who need to deal precisely with long intervals (like
astronomers and computer scientists) avoid using 'years' as a unit of
- How Long is a Meter? a Second? [09/06/2001]
How is the length of a meter currently determined?
- How Many Calendars Do I Need? [01/02/2002]
In 1980 I began collecting calendars, and I have done so every year since. What is the last year I must collect a calendar in order to have one that can be used for every year?
- How Often Are There 5 Sundays in February? [02/27/2004]
How often are there 5 Sundays during the month of February?
- Jewish Calendar [03/28/2002]
If the Jewish calendar has the same number of days each year, why do the
holidays fall at a different time each year?
- 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?
- Light Year [08/09/1997]
How long (in miles) is a light year?