Free Time Calculator — Add, Subtract & Convert Time
Add or subtract hours, minutes, and seconds. Calculate hours between two clock times, total time duration, weeks between dates, months between dates, years between dates, and elapsed time — every time calculation in one place.
Enter multiple time values to add or subtract. Add as many rows as you need.
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📚 Sources & Methodology
All time calculations on this page use these verified standards:
- ISO 8601 — International standard for date and time representation and arithmetic
- USNO (US Naval Observatory) — Official US timekeeping standards and leap second rules
- Gregorian Calendar — Month length rules, leap year correction, and date arithmetic
- US Department of Labor — FLSA overtime rules referenced in time-and-a-half explanations
- EU Working Time Directive (2003/88/EC) — Maximum work week reference for EU context
How to Calculate Time — The Complete Guide
Adding Hours and Minutes — The Core Method
Adding time is not the same as adding regular numbers because hours use base-60 for minutes and seconds. When minutes exceed 59, you carry into hours; when seconds exceed 59, you carry into minutes. This is the same logic a clock uses — and it is the most common time calculation people need for tracking work hours, event planning, and travel.
Step 1: Add all seconds together
Step 2: Carry-over = floor(total seconds / 60) → add to minutes; remainder = new seconds
Step 3: Add all minutes (including carry-over)
Step 4: Carry-over = floor(total minutes / 60) → add to hours; remainder = new minutes
Step 5: Add all hours (including carry-over)
Example: 2h 45m 30s + 1h 25m 50s = 4h 11m 20s
Subtracting Time
Subtracting time requires borrowing when the smaller unit of the result would be negative — the same concept as borrowing in regular subtraction, but across base-60 boundaries. If subtracting minutes creates a negative value, borrow 1 hour (60 minutes) from the hours column.
Method: Convert all values to total seconds, subtract, then convert back
Total seconds = (hours x 3600) + (minutes x 60) + seconds
After subtraction: hours = floor(result / 3600)
Minutes = floor((result mod 3600) / 60)
Seconds = result mod 60
Hours Between Two Times
Calculating the hours between two clock times is essential for payroll, billing, event scheduling, and work shift calculation. The key challenge is when the end time is earlier than the start time — this means the period crosses midnight and you must add 24 hours to the end time before subtracting.
Duration = End time - Start time (in total minutes)
If End time < Start time: Duration = (End time + 1440) - Start time (crosses midnight)
Example: 9:15 to 17:45 = (17x60+45) - (9x60+15) = 1065 - 555 = 510 min = 8h 30m
After deducting 30-min lunch: 8h 30m - 0h 30m = 8h 0m worked
Weeks, Months, and Years Between Dates
Time spans between calendar dates are calculated differently depending on which unit you need. Weeks are the simplest — just total days divided by 7. Months are more complex because months have different lengths. Years require checking whether the anniversary date has passed this year.
| Unit | Formula | Example (Jan 1 to Nov 15, same year) |
|---|---|---|
| Days | End - Start in calendar days | 319 days |
| Weeks | Total days / 7 | 45 weeks, 4 days |
| Months | (Y2-Y1)x12 + (M2-M1) ± day adjustment | 10 months, 14 days |
| Years | Y2-Y1, adjusted for anniversary | 0 years, 10 months, 14 days |
Time Duration for Events and Projects
For multi-day time spans — like a project timeline, a trip, or a hospital stay — you need to count both the date difference and the time component. A project starting Monday 9:00 AM and ending Wednesday 3:00 PM lasts 2 days and 6 hours. Our Time Duration tab accepts datetime inputs and calculates the exact span in days, hours, and minutes.
Calculating Work Hours and Payroll Time
The most common real-world use of a time calculator is payroll. To calculate total hours worked in a day: subtract start time from end time, then deduct any unpaid break time. For a week, sum all daily totals. Standard US law (FLSA) requires time-and-a-half pay for hours over 40 per week for non-exempt employees.
| Scenario | Calculation | Result |
|---|---|---|
| 8:30 AM to 5:00 PM, 30 min lunch | (510 - 30) min / 60 | 8.0 hours worked |
| 9:00 AM to 6:30 PM, 45 min lunch | (570 - 45) min / 60 | 8.75 hours worked |
| 40 hours regular + 5 hours OT | 40 x rate + 5 x (rate x 1.5) | 47.5 effective hours pay |
| Night shift: 10 PM to 6 AM | (360 + 1440 - 1320) min / 60 | 8.0 hours worked |
Time Zones and Cross-Timezone Duration
When calculating time durations that cross time zones — such as flight durations or international meeting times — always convert both the start and end time to UTC first, then calculate the duration, then convert back. A flight from New York (UTC-5) departing at 6:00 PM to London (UTC+0) arriving at 6:00 AM the next day is 7 hours long, not 12 hours — the time zone difference accounts for the difference.
Decimal Hours vs. Hours and Minutes
Many payroll systems and time tracking tools use decimal hours rather than hours:minutes format. Converting between formats is straightforward: decimal minutes = (minutes / 60). So 7 hours 45 minutes = 7 + (45/60) = 7.75 decimal hours. For billing at $100/hour: 7h 45m = 7.75 x $100 = $775.00. Use our Time Converter to switch between decimal and hours:minutes formats.
| Minutes | Decimal Hours | Common Use |
|---|---|---|
| 15 min | 0.25 | Quarter hour billing increment |
| 30 min | 0.50 | Half hour billing increment |
| 45 min | 0.75 | Three-quarter hour |
| 6 min | 0.10 | Legal billing (6-minute increments) |
| 20 min | 0.333 | One-third hour |