Time Calculator
Add hours and minutes to any start time to find when something ends. Subtract two clock times to find elapsed duration. Convert between all time units with full base-60 accuracy.
Enter your values above to see the results.
Tips & Notes
- ✓When adding a duration to a clock time, convert both to minutes past midnight first: 3:45 PM = 15 hours × 60 + 45 = 945 minutes. Add the duration in minutes. Convert the sum back. This eliminates AM/PM confusion and carry errors simultaneously.
- ✓For working backward (I need to arrive by X, the trip takes Y, when do I leave?): subtract the duration from the target arrival time. Departure = Arrival Time − Travel Duration.
- ✓One useful approximation: 1 year ≈ 525,600 minutes (365 × 24 × 60). This is the number from the musical Rent and makes for a memorable mental benchmark.
- ✓When computing time across time zones, convert all times to UTC first, perform the arithmetic in UTC, then convert the result back to each local time zone.
- ✓For very precise calculations (scientific or legal), distinguish between a calendar year (365 or 366 days) and a Julian year (exactly 365.25 days) and a tropical year (365.2422 days). The difference matters for multi-year spans.
Common Mistakes
- ✗Adding clock times directly instead of computing duration — 8:00 AM + 11:00 AM = 19:00 is meaningless; you can add a duration (8 hours) to a start time, but not two clock times together.
- ✗Forgetting to convert between 12-hour and 24-hour format before subtracting — subtracting 2 PM (14:00) from 8 AM (8:00) gives a negative number; subtracting 8:00 AM from 14:00 gives the correct 6-hour duration.
- ✗Using 30 days as a month and 12 months as a year for time conversion — a year has 365 or 366 days, not exactly 360. For precise calculations, use actual calendar days.
- ✗Confusing elapsed time with time of day in word problems — when does a 3h 20m process finish if it starts at 9:15 AM? This adds a duration to a time of day. The answer is 12:35 PM, not 12:35 (which would imply you are subtracting).
- ✗Treating hours in a 24-hour total as regular decimal — 28.5 hours means 1 day and 4.5 hours (not 28 hours and 50 minutes). Converting decimal hours to hours and minutes requires multiplying the decimal part by 60, not treating it as minutes directly.
Time Calculator Overview
A time calculator performs arithmetic on time values — adding hours and minutes to find a future time, subtracting time to find an elapsed duration, or converting between time units including seconds, minutes, hours, days, weeks, and years. Unlike ordinary arithmetic, time calculations operate in mixed bases: 60 seconds per minute, 60 minutes per hour, 24 hours per day, 7 days per week — a system that requires careful carry and borrow operations that ordinary calculators handle incorrectly.
Adding time to find a future time:
Future Time = Start Time + Duration | carry minutes into hours if sum ≥ 60, wrap hours if sum ≥ 24
EX: Meeting starts at 10:45 AM, runs 2 hours 40 minutes → 10:45 + 2:40 → hours: 12, minutes: 85 → 85−60=25, hours: 13 → End time: 1:25 PMSubtracting time to find elapsed duration:
EX: Start 9:15 AM, end 4:50 PM → 4:50 PM = 16:50 → 16:50 − 9:15 → hours: 7, minutes: 35 → Duration: 7h 35mTime unit conversion reference:
| Unit | Equals | In Seconds | Common Use |
|---|---|---|---|
| 1 second | 1 second | 1 | Stopwatch, reaction time |
| 1 minute | 60 seconds | 60 | Timers, cooking |
| 1 hour | 60 minutes | 3,600 | Work, travel |
| 1 day | 24 hours | 86,400 | Calendar, deadlines |
| 1 week | 7 days | 604,800 | Scheduling, notice periods |
| 1 month (avg) | 30.44 days | 2,629,800 | Billing cycles (approximate) |
| 1 year (common) | 365 days | 31,536,000 | Annual calculations |
| 1 year (average) | 365.2425 days | 31,556,952 | Astronomical calculations |
| Expression | Hours | Minutes | Seconds | Decimal Hours |
|---|---|---|---|---|
| 1 day, 3 hours, 20 min | 27 | 1,620 | 97,200 | 27.333 |
| 5 hours, 45 minutes | 5.75 | 345 | 20,700 | 5.750 |
| 2.5 hours | 2.5 | 150 | 9,000 | 2.500 |
| 90 minutes | 1.5 | 90 | 5,400 | 1.500 |
Frequently Asked Questions
Convert both to minutes past midnight, add, then convert back. Example: 2:45 PM + 3h 20m. Convert: 2:45 PM = 14:45 = (14 × 60) + 45 = 885 minutes. Add duration: 3 hours 20 min = 200 minutes. 885 + 200 = 1085 minutes. Convert back: 1085 ÷ 60 = 18 hours remainder 5 min = 18:05 = 6:05 PM. Alternatively: add hours first (14+3=17), add minutes (45+20=65), carry 60 min to hour: 18:05.
A common year: 365 × 24 × 60 × 60 = 31,536,000 seconds. A leap year: 366 × 24 × 60 × 60 = 31,622,400 seconds. The average Gregorian year (accounting for leap year frequency) = 365.2425 × 86,400 = 31,556,952 seconds. For a rough mental approximation: π × 10^7 ≈ 31,415,927 — the classic approximation that gives about 0.38% error.
Multiply hours by 60 and add the remaining minutes. Example: 3 hours 45 minutes = (3 × 60) + 45 = 180 + 45 = 225 minutes. To convert back: divide total minutes by 60 — the integer quotient is the hours and the remainder is the minutes. 225 ÷ 60 = 3 remainder 45 = 3 hours 45 minutes.
Subtract the start time from the end time, using 24-hour format to avoid AM/PM issues. If both times are on the same day: 16:30 − 09:45 → hours: 6, minutes: 45 → 6h 45m. If minutes require borrowing: 16:15 − 09:45 → hours: 6, minutes: −30 → borrow: hours 5, minutes 30 → 5h 30m. If the span crosses midnight, add 24 hours to the end time before subtracting: 06:00 next day = 30:00 → 30:00 − 22:30 = 7:30 hours.
Subtract the travel duration from the target arrival time. Example: arrive by 2:00 PM (14:00), trip takes 2h 45m. Departure = 14:00 − 2:45 → hours: 11, minutes: 60−45=15, borrow 1 hour: 11:15 AM. Add buffer for traffic or delays — if you need 30 minutes extra buffer: 11:15 − 0:30 = 10:45 AM departure.
A clock time is a specific point in the day (3:30 PM, 08:45). A duration is an elapsed interval with no fixed reference point (2 hours 15 minutes, 90 minutes). You can add a duration to a clock time to get another clock time. You can subtract two clock times to get a duration. You cannot add two clock times together — only a clock time and a duration. Confusing these two concepts is the root of most time arithmetic errors.