Start time calculator
Calculate a start time from an end time and a duration.
Automatically calculate the required start time from an end time and a duration to subtract. Set the duration on the left, the end time on the right, and get the starting time instantly.
Calculation data
Duration to subtract
End time
Results
| Start time | |
|---|---|
| Day offset | |
| Subtracted duration | |
| H:M:S format | |
| Subtracted seconds | |
| Selected end |
Quick explanation
Enter a duration in H:M:S format, then an end time in HH:MM:SS format. The tool subtracts the selected duration from the end time and displays the start time, with a possible offset to the previous day or several days earlier.
Try this too
Need the reverse calculation?
Use the End time calculator to add a duration to a start time and get the finishing time.
Additional information
Find when an activity should start
This tool answers a very common question: if something must end at a specific time and lasts for a given duration, when should it start? Enter the duration to subtract and the desired end time.
The calculation updates automatically. You can adjust hours, minutes and seconds until the starting time matches your schedule.
Duration to subtract and end time
The duration to subtract is an interval. It can be short or long, such as 30 minutes, 2 hours 15 minutes or several hours. The end time is a precise clock time in HH:MM:SS format.
This distinction matters: the end time stays within a 24-hour clock format, while the duration is used only to move backward in time.
When the result goes before midnight
If the subtracted duration is longer than the end time, the result may fall on the previous day or several days earlier. The tool normalizes the displayed time and shows the day offset to avoid confusion.
This is useful for long trips, preparation times, cooking delays, rest periods or tasks that must be completed by a specific deadline.
Practical examples
Use it to know when to start preparing for a meeting, calculate a departure time before an appointment, plan the start of a cooking step, organize a work session or check when a task should begin before a deadline.
For the reverse calculation, when you already know the start time and duration, use the End time calculator instead.
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