Time Addition/Subtraction Calculator

Time arithmetic involves adding, subtracting, and converting between hours, minutes, and seconds. Unlike decimal numbers, time uses base-60 for seconds and minutes and base-24 for hours, so a plain calculator gets it wrong unless you convert first.

Key conversion factors:

• 1 hour = 60 minutes = 3,600 seconds
• 1 day = 24 hours = 1,440 minutes = 86,400 seconds
• 1 week = 7 days = 168 hours

Adding times: 3 hours 45 minutes + 2 hours 30 minutes: Minutes: 45 + 30 = 75 → 1 hour 15 minutes carry-over Hours: 3 + 2 + 1 = 6 Result: 6 hours 15 minutes

Subtracting times: 5 hours 20 minutes − 2 hours 50 minutes: Borrow 60 minutes: 5h 20min becomes 4h 80min 80 − 50 = 30 minutes 4 − 2 = 2 hours Result: 2 hours 30 minutes

Converting decimal hours to hours and minutes: 3.75 hours = 3 hours + (0.75 × 60) minutes = 3 hours 45 minutes

Converting minutes to decimal hours: 2 hours 20 minutes = 2 + (20/60) = 2.333… hours

The decimal form is the one payroll systems want, and it is where most hand-calculated timesheets go wrong. 7 hours 45 minutes is 7.75 hours, not 7.45. Anyone typing 7.45 into a spreadsheet has just given away 18 minutes.

Worked example, a time sheet calculation: Start: 08:45, End: 17:15, Lunch break: 45 minutes Total elapsed: 17:15 − 08:45 = 8 hours 30 minutes Less lunch: 8h 30min − 45min = 7 hours 45 minutes = 7.75 decimal hours

24-hour vs 12-hour clock: 14:30 (24-hour) = 2:30 PM (12-hour) To convert 24-hour afternoon times: subtract 12 (14:30 − 12 = 2:30 PM)

Time zone arithmetic: UTC+2 is 5 hours ahead of UTC−3. If it is 10:00 in UTC−3, it is 15:00 in UTC+2.

Time arithmetic is used in payroll, scheduling, travel planning, sports timing, and project management.