Adding and Subtracting Time: How to Calculate Hours and Minutes

Why is adding "1 hour 45 minutes" to "2 hours 30 minutes" so much harder than adding 1.75 + 2.50? Because time doesn't follow the rules of normal math.

The Base 60 Problem

Most of our math is Decimal (Base 10). When you reach 10, you carry the 1.

Time typically uses Sexagesimal (Base 60). You have to wait until you reach 60 minutes to "carry the 1" over to the hour column. This mental switching is what makes calculating timesheets or flight durations so prone to error.

How to Add Time Manually

Let's add 2h 45m and 1h 35m.

1. Add the Minutes: 45 + 35 = 80 minutes.
2. Convert to Hours: Since 80 > 60, we subtract 60 minutes (1 hour) and are left with 20 minutes. We have 1 extra hour to carry.
3. Add the Hours: 2 + 1 + 1 (carry) = 4 hours.

How to Subtract Time Manually

Let's subtract 1h 50m from 3h 10m.

1. Try Subtracting Minutes: 10 - 50... we can't do that without getting a negative number.
2. Borrow an Hour: Take 1 hour from the 3h, leaving 2h. Convert that hour into 60 minutes and add it to the 10m. Now we have 70 minutes.
3. Subtract Minutes: 70 - 50 = 20 minutes.
4. Subtract Hours: 2 - 1 = 1 hour.

Military Time vs. Standard Time

Another layer of complexity is the AM/PM system.

• Standard Time: Repeats 1-12 twice a day. Calculating duration across noon or midnight is tricky (e.g., 10:00 AM to 2:00 PM).
• Military Time (24-hour): Ranges from 00:00 to 23:59. This makes math easier. 10:00 to 14:00 is obviously 4 hours.

Conclusion

Time is our most valuable resource, so don't waste it doing Base 60 math on a napkin. Whether you're tracking payroll or planning a movie marathon, our Time Calculator keeps you on schedule.