What this result means
Enter the time of your last calories and the time of your next calories to calculate the window. You can also start with a time and duration to calculate an end time.
What the result means
In duration mode, the main result is the exact clock-time gap between the two entries. When the end time is earlier, it is treated as the following day.
In end-time mode, the main result is the local clock time reached after adding the exact duration you entered. The +1 day label makes a midnight rollover explicit.
The result does not say whether that window is suitable, beneficial or safe for you. [1]
What may affect the calculation
Use the clock time when calories actually stopped or resumed, not the time a meal was planned. Drinks, supplements or other products may contain calories, but this tool does not classify what counts for your purpose.
A clock-only calculation cannot detect daylight-saving transitions or travel between time zones. Use dated, zone-aware records when either occurred.
The remaining daily window is only the mathematical complement to 24 hours. It is not a recommended eating period.
Safe, practical next steps
Check that you used the correct side of midnight and recalculate if either time or duration was entered incorrectly.
Do not use this arithmetic to change medicines or manage a health condition. Healthdirect notes that intermittent fasting is not suitable for everyone and advises medical guidance for people with health conditions or medicines that may be affected. [1]
If you need personal advice about whether or how to fast, use a qualified health professional rather than interpreting this result as guidance. [1]
Why there are no fasting protocols or stages
Labels such as 16:8 describe a schedule, but a clock result alone cannot establish health effects, nutritional quality or individual suitability. [1]
This tool therefore does not recommend protocols, predict metabolic stages, estimate ketosis or autophagy, or turn a longer duration into a better score.
Private by design, limited by design
The calculation runs in your browser. Times, durations and results are not placed in the page URL, the shared link or analytics. Sharing sends only this tool's public link.
The tool does not ask for age, weight, diagnoses, medicines, pregnancy, goals or any other health detail.
Formula and assumptions
- Formula
- Elapsed: F = E − S when E > S; otherwise F = E + 1440 − S. Planned end: E = (S + D) mod 1440, with +1 day when S + D ≥ 1440. Remaining daily window: R = 1440 − F.
- Version
- Neutral fasting-window clock arithmetic, fasting-window-neutral-v1
- Rounding
- Convert each HH:MM entry and each whole-hour or whole-minute duration to exact integer minutes. Display exact hours and minutes without decimal rounding. Equal elapsed-mode times are rejected as ambiguous; planned duration must be 1 to 1,440 minutes.
- Worked example
- Example: a window starting at 20:00 and ending at 12:00 the next day is exactly 16:00. The remaining time in that 24-hour cycle is 8:00. In planning mode, starting at 20:00 and adding 16 hours produces 12:00 (+1 day).
Who this is for — and when not to use it
Intended use
Neutral calculation of one fasting-related clock interval lasting no more than 24 hours, using times or a duration the user already chose.
Not intended for
Not fasting advice, a timer, a protocol selector, a health or adherence score, a measure of physiology, a nutrition plan, or guidance about food, fluids, medicines, weight loss, disease, pregnancy, religious practice or treatment.
Exclusions
- Equal start and end times in elapsed mode, because zero versus 24 hours is ambiguous
- Intervals longer than 24 hours or multiple fasting periods
- Daylight-saving clock changes and travel across time zones
- Recommended fasting durations, protocols, targets, readiness or adherence scoring
- Weight-loss, calorie, macronutrient, metabolic-stage, ketosis or autophagy estimates
- Medication timing, diabetes management, pregnancy, breastfeeding, eating disorders, childhood or clinical decisions
- Religious-law, occupational, driving, emergency or treatment decisions
- Timers, reminders, streaks, alarms and live countdowns
Evidence sources
- 1. Intermittent fasting — Healthdirect Australia. Accessed Sep 3, 2026.