Calculating Time and Duration

Master time math with just your brain, paper, and a basic calculator.

Whether you’re planning a trip, logging work hours, or figuring out how long it takes to cook a roast, time calculations are a part of everyday life. While clocks and apps are helpful, there are plenty of situations where a basic calculator is all you need to get the job done.

This guide walks you through:

• Adding and subtracting hours and minutes

• Converting between 12-hour and 24-hour formats

• Planning using time math for schedules and deadlines

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🕒 1. Adding and Subtracting Hours and Minutes

The Key Rule:

There are 60 minutes in an hour—not 100! So you can’t treat time like regular decimals.

Let’s walk through how to properly handle time math with a calculator.

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✅ Example 1: Adding Time

Start time: 2 hours 45 minutes
Add: 1 hour 50 minutes

Step 1: Convert both to total minutes
→ 2 hr 45 min = (2 × 60) + 45 = 165 min
→ 1 hr 50 min = (1 × 60) + 50 = 110 min

Step 2: Add them together
→ 165 + 110 = 275 min

Step 3: Convert back to hours and minutes
→ 275 ÷ 60 = 4.5833
→ 4 full hours, remainder: 275 − 240 = 35 min
Final answer: 4 hours 35 minutes

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✅ Example 2: Subtracting Time

Start: 3 hr 15 min
Subtract: 1 hr 40 min

Step 1: Convert to minutes
→ 195 − 100 = 95 min
Step 2: Convert 95 min back
→ 95 ÷ 60 = 1.5833 = 1 hour, 35 minutes
Final answer: 1 hr 35 min

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🌓 2. Converting Between Time Formats

You’ll often need to switch between 12-hour AM/PM time and 24-hour military time, especially for travel, scheduling, or work.

🕗 12-hour to 24-hour:

✅ Tip: Add 12 to the PM hour (except for 12 PM itself)

🕛 24-hour to 12-hour:

• If the hour is 13 or more, subtract 12 and add PM
  → 18:45 → 18 − 12 = 6:45 PM

• If under 12, it’s AM (unless it’s 12:00 = noon)

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📆 3. Planning with Time Calculations

Example: Scheduling a Meeting

Start: 9:40 AM
Duration: 2 hr 35 min

Step 1: Convert start to minutes
→ 9 × 60 + 40 = 580
Step 2: Add duration
→ 580 + 155 = 735
Step 3: Convert 735 min to hours
→ 735 ÷ 60 = 12.25 → 12 hr 15 min
Meeting ends at: 12:15 PM

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Example: Total Work Hours

Day 1: 8:30 AM to 12:45 PM =

• 12:45 − 8:30 = 4 hr 15 min

Day 2: 9:00 AM to 5:00 PM (with 1 hr lunch)

• 8 hrs − 1 = 7 hrs

Add both days:
→ 4 hr 15 min + 7 hr = 11 hr 15 min

Use calculator:

• Convert to mins: 255 + 420 = 675

• 675 ÷ 60 = 11.25 → 11 hours 15 minutes total

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🧮 Bonus: Decimal Time vs. Clock Time

Sometimes you'll see time reported as decimals, especially in work logs or invoices.

Examples:

• 1.5 hours = 1 hr 30 min

• 2.25 hours = 2 hr 15 min

• 3.75 hours = 3 hr 45 min

To convert decimal to hours and minutes:

1. Take the decimal part (e.g. .75)

2. Multiply by 60
   → .75 × 60 = 45 min

So: 3.75 hr = 3 hr 45 min

To go backwards: → 45 min = 45 ÷ 60 = .75
→ 2 hr 15 min = 2 + (.25) = 2.25

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⏱️ Final Thoughts

Understanding how to work with time is essential—and you don’t need a special app or advanced watch to do it. Just convert hours and minutes properly, keep 60-minute hours in mind, and let your calculator do the heavy lifting.

Quick Recap:

• Always convert hours and minutes to total minutes before adding or subtracting

• Convert 24-hour time to 12-hour format by subtracting 12 from any hour past noon

• For decimals, multiply or divide by 60 to move between hours and minutes