What is the Reverse Time Calculator?
The Reverse Time Calculator works backwards from a finish time. If you know when something must end and how long it takes, this tool tells you exactly when to start. It is perfect for cooking, meeting prep, travel, exam timing, and shift planning. The calculator uses a standard 24-hour clock and automatically handles cases that roll back past midnight into the previous day.
How to use it
Enter the end time as an hour (0–23) and a minute (0–59). Then enter the duration you want to subtract as hours and minutes. The result shows the start time in HH:MM format, along with a day offset that tells you if the start falls on a previous day.
The formula explained
Everything is converted to minutes since midnight. The end time in minutes is \(E = E_h \times 60 + E_m\), and the duration is \(D = D_h \times 60 + D_m\). The raw start is \(S = E - D\). To keep the answer on a valid clock, we apply $$((S \bmod 1440) + 1440) \bmod 1440$$ which wraps negative values forward into the prior day. A separate day-offset value records how many midnights were crossed.
Worked example
Suppose a roast must be ready at 18:30 and takes 2 hours 45 minutes. End = \(18 \times 60 + 30 = 1110\) minutes. Duration = \(2 \times 60 + 45 = 165\) minutes. Start = \(1110 - 165 = 945\) minutes = 15:45. So you should begin at 3:45 PM, same day.
FAQ
What happens if the start is before midnight? If subtracting pushes the time below 0, the calculator wraps it to the correct hour and reports a day offset of \(-1\) (previous day).
Does it use 12-hour or 24-hour time? Input and output use 24-hour time. 13:00 is 1:00 PM, and 00:00 is midnight.
Can I subtract more than 24 hours? Yes. Enter any number of hours; the day offset will show how many full days back the start time falls.
