Reverse Time Calculator — Find Start or End Time
Know when something must end and how long it takes? Work backwards to find the start time — or given a start time and duration, calculate when it will finish.
What to calculate
h
min
h
min
- 1
End time in minutes
9 × 60 + 0 = 540 - 2
Duration in minutes
1 × 60 + 30 = 90 - 3
Calculated start time
540 − 90 = 07:30
How does this calculator work?
Start = End − Duration; End = Start + Duration. All arithmetic is in minutes, wrapping past midnight automatically. Enter a known time and duration to get the other time displayed as HH:MM on a 24-hour clock face and a timeline.
Formula
How this is calculated
The calculator converts all inputs to total minutes — hours × 60 plus minutes — then adds or subtracts the duration depending on the selected mode. Working backwards ("Find start time") answers questions like "My flight departs at 15:00 and I need 2 hours 30 minutes to get ready — what time do I need to start?" Adding forward ("Find end time") answers "I start at 08:30 and the task takes 1 hour 45 minutes — when will I finish?"
Because time wraps around midnight, the result is normalised to the range 00:00–23:59 using modulo 1 440 (total minutes in a day). An overnight calculation — such as subtracting 3 hours from 01:00 — correctly produces 22:00 the previous evening. The clock face shows the calculated time visually, and the timeline bar shows the duration as a fraction of the full 24-hour day.
All inputs and outputs use the 24-hour clock (00:00–23:59). To convert from 12-hour AM/PM format: add 12 to any PM hour (except 12 PM, which stays 12:00), and use 00:00 for 12 AM midnight.
Frequently asked questions
Select "Find start time", enter the end time (in 24-hour format) and the duration in hours and minutes. The calculator subtracts the duration and shows the start time, wrapping past midnight automatically.
The result wraps around automatically. For example, if the end time is 01:00 and the duration is 3 hours, the start time is shown as 22:00 (the previous evening). The clock and timeline are normalised to a single 24-hour day.
Yes — any duration is supported. The displayed time shows the equivalent position on the 24-hour clock. For example, a 25-hour duration from 08:00 gives 09:00 (displayed as 09:00 on the clock face).
Also known as
TG we-Calculate Editorial Team. (2026). Reverse Time Calculator — Find Start or End Time [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/reverse-time-calculator
TG we-Calculate Editorial Team. "Reverse Time Calculator — Find Start or End Time." TG we-Calculate. 2026. https://we-calculate.com/calculator/reverse-time-calculator.
TG we-Calculate Editorial Team, "Reverse Time Calculator — Find Start or End Time," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/reverse-time-calculator
@misc{wecalculate_reverse_time_calculator, title = {Reverse Time Calculator — Find Start or End Time}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/reverse-time-calculator}}, year = {2026}, note = {TG we-Calculate} }
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