Backwards timing is the everyday version of this sum: if something must be finished by 17:30 and it takes 6:45, work has to begin at 10:45. The calculator converts the finish time to minutes past midnight, subtracts the duration, and wraps into the previous day when the result goes below zero, so the answer is always a real clock time.
The previous-day case is the one worth watching. A twelve-hour job finishing at 07:00 has to start at 19:00 the day before, and a result that silently lost a day is the sort of error that only shows up when nobody turns up. The extras name the day explicitly rather than leaving it implied.
The midpoint is given as well, since it is the natural point to check progress against. It sits half the duration after the start, and for a long job it is often more useful than either end.
The formula
F- Finish time, minutes past midnight
D- Duration in minutes
S- Start time
M- Midpoint of the block
How it works, step by step
- Enter the time the task must be finished by.
- Enter how long the task takes, as H:MM or a decimal number of hours.
- Read the start time in both 24-hour and 12-hour form.
- Check whether the start falls on the previous day.
Worked examples
Finish by 17:30, 6:45 of work
1050 minutes past midnight less 405 minutes leaves 645, which is 10:45 — the same day. The halfway point falls at 14:07.
A twelve-hour job finishing at 07:00
420 minus 720 minutes is negative, so a day is added back: the start is 19:00 on the previous day. The midpoint sits at 01:00, just after midnight.
How to read your score
Frequently asked questions
What if the start lands on the previous day?
The calculator wraps the clock and says so in the extras. It never returns a negative time, and it names how many days back the start falls so a date can be adjusted alongside it.
Is this the same as the time duration calculator?
No. That page takes two clock times and returns the gap; this one takes a clock time and a gap and returns the other clock time. They solve the same triangle from different corners.
Can the duration be longer than a day?
Yes. Enter it as a decimal — 30 means thirty hours — and the result will report that the start falls more than one day earlier.
Why show a 12-hour version as well?
Because rotas and public-facing schedules are usually written with am and pm, while payroll and planning systems use 24-hour time. Showing both avoids a transcription error between the two.
Backward timing reference
| Finish by | Length | Start at | Same day? | Midpoint |
|---|---|---|---|---|
| 17:30 | 6:45 | 10:45 | Yes | 14:07 |
| 12:00 | 3:30 | 08:30 | Yes | 10:15 |
| 09:00 | 2:15 | 06:45 | Yes | 07:52 |
| 08:00 | 10:00 | 22:00 | Previous day | 03:00 |
| 07:00 | 12:00 | 19:00 | Previous day | 01:00 |
| 02:00 | 5:30 | 20:30 | Previous day | 23:15 |
The last three rows start on the previous day, which is why the clock wraps.