Why is Calculating Time So Difficult?
If you have ever tried to add 2 hours and 45 minutes to 3 hours and 30 minutes in your head, you know it gets confusing quickly. This is because standard mathematics is based on the decimal system (Base-10), but time is based on the sexagesimal system (Base-60).
In normal math, 50 + 50 = 100. But in time math, 50 minutes + 50 minutes does not equal 100 minutes—it equals 1 hour and 40 minutes. Because time rolls over at the number 60 instead of 100, regular calculators cannot process time correctly.
How to Manually Add or Subtract Time
If you do not have a time calculator handy, here is the secret to doing time math manually on paper:
Step 1: Add the hours together. (2 + 1 = 3 hours)
Step 2: Add the minutes together. (45 + 35 = 80 minutes)
Step 3: If the total minutes are 60 or higher, subtract 60 from the minutes and add 1 to the hours.
(80 - 60 = 20 minutes left over. Add 1 to the hours: 3 + 1 = 4 hours).
Final Answer: 4 hours and 20 minutes.
Handling Overnight Shifts (Time Differences)
One of the most common issues people face is calculating the hours worked on an overnight shift. For example, starting work at 10:00 PM and ending at 6:00 AM. If you subtract 10 from 6, you get a negative number.
Example: Start 22:00 (10 PM), End 06:00 (6 AM).
Instead of 6 - 22, do (6 + 24) - 22 = 8 Hours.
Decimal Hours vs Clock Hours
If you are calculating time for a paycheck, do not confuse clock time with decimal time. 8 hours and 30 minutes does not equal 8.30. Because 30 minutes is exactly half of an hour, it translates to 8.50 in decimal format. To convert minutes to decimals manually, divide the minutes by 60.