Planning backwards from a fixed deadline – dinner at 7 pm, a flight, an exam – is more reliable than planning forwards. Breaking the job into steps and adding a buffer of 10–20% avoids running late.
How to use the Reverse Time Calculator – When to Start
- Finish by
- Step 1
- Step 2
- Step 3
- Buffer
Results update as you type, and every field accepts the units shown in its menu.
How it works
Pick the time you need to be finished, then enter the length of each step and a buffer.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Finish by | A clock time (hours and minutes) |
| Step 1 | In min |
| Step 2 | In min |
| Step 3 | In min |
| Buffer | In min |
What each result means
- Total time needed = ([Step 1] + [Step 2] + [Step 3] + [Buffer])÷60
- Start at = [Finish by] – [Total time needed]
- Start at (24-hour) = [Finish by] – [Total time needed]
- Step 2 starts = [Finish by] – [Total time needed] + [Step 1]÷60
- Step 3 starts = [Finish by] – [Total time needed] + ([Step 1] + [Step 2])÷60
Formula
Start time = finish time − (step 1 + step 2 + step 3 + buffer)Worked example
Inputs — Finish by: 19:00; Step 1: 25 min; Step 2: 90 min; Step 3: 15 min; Buffer: 15 min.
Results — Total time needed = 2 h 25 min; Start at = 4:35 PM; Start at (24-hour) = 16:35; Step 2 starts = 5:00 PM; Step 3 starts = 6:30 PM; Start day = Same day.
Frequently asked questions
When should I start cooking to eat at 7 pm?
Add up prep, cooking and resting time plus a buffer, and subtract it from 7 pm.
How much buffer should I add?
About 10–20% of the total, more for travel.
Can I plan across midnight?
Yes – the result shows if you need to start the day before.
What formula does the Reverse Time Calculator – When to Start use?
Start time = finish time − (step 1 + step 2 + step 3 + buffer).