How to Calculate Time

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Mathematics, Measurement (includes Time)

Grade 3- 5


Students learn to calculate time with clocks, calendars, and eras.


Give students the following information about computing time with clocks, calendars, and eras.
Facts to Know
a.m. = morning - 12:00 a.m. (midnight) to 11:59 a.m.
p.m. = afternoon - 12:00 p.m. (noon) to 11:59 p.m.
60 seconds = 1 minute
60 minutes = 1 hour
24 hours = 1 day
To compute elapsed time within the morning or within the afternoon, subtract the smaller number from the larger number. Remember to regroup (borrow) with 60 minutes.
Because you cannot subtract 0 minutes from 38 minutes, subtract 1 hour from the 9:00 p.m. and convert it to 60 minutes. So when you calculate how much time elapsed between 9:00 p.m. and 7:38 p.m., the final answer is 1 hour 22 minutes.
9:00 p.m. (or 8:60min) - 7:38 p.m. = 1:22 (or 1 hr 22 min)
To add elapsed time, add the two measurements of time together.
When you add the two measurements of time together, you have an answer of 218 days 49 hrs 60 min, but you know that you convert some of the minutes to hours and some hours into days. Using the chart at the top of the page, you know that 24 hours = 1 day so 49 hours = 2 days 1 hour. Similarly, you know that 60 minutes = 1 hour so after you made all the conversions, the final answer is 220 days and 2 hours have elapsed.
206 days 2 hrs 5 min + 12 days 47 hrs 55 min = 218 days 49 hrs 60 min (or 220 days 2 hrs)
Calendar Facts
7 days = 1 week
52 weeks = 1 year
10 years = 1 decade
10 decades = 1 century
10 centuries = 1 millennium
28-Day Month
February (29 days in leap year)
30-Day Months
31-Day Months
Time from the approximate date of the birth of Christ until the present moves progressively from 1 to 2000 +. It is called A.D. (anno domini--in the year of our Lord).
Time before the birth of Christ counts back from 1 to the earliest recorded history, about 5,000 years. It is called B.C. (before Christ) or B.C.E. ("before the common era").
To compute the passage of years within B.C. or within A.D., subtract the lower number from the higher number.
To compute the passage of years from B.C. to A.D., add the B.C. date to the A.D. date.


  • Using a Calendar activity sheet
  • Computing Time Before and After the Present Era activity sheet

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