What is Time Duration Calculator?
Time Duration Calculator finds the exact duration between two times — in hours, minutes, seconds, and decimal hours. It handles overnight spans, lets you add durations together for total time tracking, and converts between all time units.
Manual time math is surprisingly error-prone — especially across midnight or when adding multiple sessions. The calculator handles all edge cases so you bill accurately and track time correctly.
Modes and Features
Duration Between Times
Enter start and end time (HH:MM) and get the exact gap — handles overnight spans automatically.
Add / Subtract Time
Start with a time and add or subtract hours and minutes to find the resulting time.
Session Running Total
Log multiple start/end pairs and get the cumulative total — ideal for timesheet use.
Decimal Hours
Every duration is shown in both HH:MM and decimal format (1.5 = 1h 30m) for billing.
How to Use Time Duration Calculator
Enter start time
Type or pick the start time in HH:MM format (24-hour or 12-hour with AM/PM).
Enter end time
Type or pick the end time. Enable 'Next day' if the end time is past midnight.
See the duration
Hours, minutes, seconds, and decimal hours appear instantly.
Add more sessions
Click 'Add session' to log another time block — the running total updates automatically.
Time Unit Conversion Reference
| Duration | Hours | Minutes | Seconds |
|---|---|---|---|
| 15 minutes | 0.25 h | 15 min | 900 s |
| 30 minutes | 0.5 h | 30 min | 1,800 s |
| 1 hour | 1.0 h | 60 min | 3,600 s |
| 1.5 hours | 1.5 h | 90 min | 5,400 s |
| 8 hours | 8.0 h | 480 min | 28,800 s |
Tips & Common Mistakes
Use decimal hours for freelance invoicing. Clients pay on decimal hours — 1h 45m billed at $100/h is 1.75 × $100 = $175. Always convert to decimal before calculating the invoice.
Mark “next day” for overnight sessions. If you worked from 10 PM to 1 AM, the calculator needs to know the end time is the next day. Without this flag, it would calculate -21 hours instead of +3.
Log in real time. Don’t rely on memory for when you started. Bookmark the calculator and enter start times as you begin each session.
Related Tools
- Date Difference — calculate days, weeks, or months between two dates
- Business Days Calculator — count working days excluding weekends
- Epoch Converter — convert timestamps to check exact durations