Fasting schedule calculator
Find the end date and time for a fast of any length from 1 to 24 hours.
Calculate from a date and time
Planning note
Times use your device’s local time zone. If the clocks change for daylight saving time, the end is calculated from real elapsed hours, so the clock time can look one hour different. See daylight saving time and travel.
How to use the schedule calculator
- Enter the date and time you start, or keep the current time that is filled in for you.
- Enter a duration in whole hours from 1 to 24.
- Read the end date and time. Press “Use this duration in the timer” to start the timer with the same length.
Worked examples
| Start | Duration | Ends |
|---|---|---|
| Monday 20:00 | 12 hours | Tuesday 08:00 |
| Monday 20:00 | 16 hours | Tuesday 12:00 |
| Friday 19:30 | 14 hours | Saturday 09:30 |
| Sunday 21:00 | 18 hours | Monday 15:00 |
To work it out by hand, add the hours to the start time and subtract 24 if the total passes midnight; the end is then on the next day. For 20:00 plus 16 hours: 20 + 16 = 36, and 36 − 24 = 12, so 12:00 the next day.
Why the end time can look one hour off
The calculator counts real elapsed hours. On the night the clocks change for daylight saving time, 16 elapsed hours can end at a clock time one hour earlier or later than simple addition suggests. That is correct: the fast is still 16 hours long. The same applies when you travel across time zones. See daylight saving time and travel.
Planning ahead
Knowing the end time in advance makes it easier to plan a meal, a workout or a family dinner around it. If your end time lands at an awkward hour, such as during your commute, try moving the start earlier or later. For a week with different start times on different days, use the weekly planner.
Medical fasting is different
If a doctor, lab or hospital has asked you to fast before a blood test, scan or procedure, follow their instructions exactly, including what you may drink and when to take medicines. This calculator does not know those rules.
Reviewed October 2026 by Lee Palmer · How we write our guides · Report an error