resetTo calculate a planned fasting endpoint, add the target duration to the actual start. To calculate a completed interval, subtract the start from the end, keeping the dates. Those are related calculations, but they answer different questions.
Add hours across midnight
Suppose your start is Monday at 7:45 pm and the target is 14 hours. There are four hours and 15 minutes until midnight. Subtract that from 14 hours, leaving nine hours and 45 minutes. The endpoint is Tuesday at 9:45 am.
The schedule calculator does this for a regular 24-hour day. Its result includes “tomorrow” when the interval crosses midnight.
Work backwards from a meal
If a planned first meal is 10 am and the interval is 14 hours, subtract 10 hours to reach midnight, then another four to reach 8 pm the previous evening.
This reverse calculation is useful when work fixes your morning meal. If the resulting evening time is impractical, the timetable needs reconsidering. The arithmetic is not an instruction to skip a meal.
Calculate what actually happened
A recorded interval from 8:20 pm Monday to 9:50 am Tuesday is 13 hours and 30 minutes:
- 8:20 pm to midnight is three hours and 40 minutes.
- Midnight to 9:50 am is nine hours and 50 minutes.
- The sum is 13 hours and 30 minutes after carrying 60 minutes into one hour.
In decimal hours, that is 13.5, not 13.30. Half an hour is half of 60 minutes. Mixing these formats is a common spreadsheet error.
Know the limits of simple clock arithmetic
Time-zone travel and daylight-saving changes can make a wall-clock subtraction differ from elapsed time. For those cases, keep dates and time zones and see the clock-change guide.
A calculated duration says nothing about whether fasting is suitable for you or whether a physiological event has occurred. Review the schedule safety context before changing your routine. In Reset, use actual start and end times; the target should describe the plan, not overwrite the result.