Everyday & Conversions
Reverse Time Calculator
Work backwards from a finish time: what time do you need to start if the task takes a set number of hours and minutes?
Your inputs
Change any value to update the result instantly.
Start at
2:15 PM
24-hour clock
14:15
Finish time
6:00 PM
Task length
3.75 hours
Day
Same day
What this calculator does
Works backwards from a finish time to the required start time.
What your result means
The clock time you need to begin, and whether that falls on the previous day.
Formula and working
Start time = finish time − duration (wrapping across midnight)
- 1
Collect the inputs
Finish hour (24 h) = 18 h; Finish minute = 0 min; Task length — hours = 3 h; Task length — minutes = 45 min
- 2
Apply the formula
Start time = finish time − duration (wrapping across midnight)
- 3
Result
Start at = 2:15 PM
Frequently asked questions
Symbols and assumptions
- t₂
- Finish time
- Δt
- Task length
- • Times are on a 24-hour clock and wrap across midnight.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Travel, setup and buffer time are not added automatically.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
Report a mistake or request an improvement
Tell us if a formula, unit, explanation, or result needs attention. Your message will include this calculator’s name and page.
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