How to use the business hours deadline calculator
- Enter the start date and time and the number of working hours to add. Decimals work, so 7.5 is seven and a half hours.
- Set when the work day starts and ends and choose the working days: Monday to Friday, Monday to Saturday or every day.
- Open the advanced options to add an unpaid break inside the day and a list of days off as YYYY-MM-DD dates.
- Read the deadline and the table showing which hours fall on which day.
Formula
Walk forward from the start time through each working window
Window = work day start to end, minus any break
Skip non-working weekdays and listed days off
Count whole minutes until the total equals the hours to add x 60
If the start time is outside working hours, counting begins at the next time work opens. A deadline that lands exactly at closing time is shown at closing time.
Twenty working hours from a Wednesday afternoon
Start on Wednesday, November 4, 2026 at 3:30 PM with a 9:00 AM to 5:00 PM Monday to Friday schedule. Wednesday gives 1.5 hours, Thursday 8 and Friday 8, which makes 17.5. The last 2.5 hours fall on Monday, November 9, from 9:00 AM, so the deadline is Monday at 11:30 AM.
Starting on Saturday, November 7 at 10:00 AM with 3 hours to add counts from Monday 9:00 AM and ends at 12:00 PM on Monday, November 9. With November 26 and 27 marked as days off, 10 hours from Wednesday, November 25 at 3:00 PM uses 2 hours that day and 8 on Monday, November 30, ending at 5:00 PM.
Hours are not the same as calendar time
A promise of a reply within 24 business hours is not a promise of a reply tomorrow. Nights, weekends and holidays do not count, so a request received late on Friday may not be due until the middle of the next week. Counting the hours day by day removes the guesswork and shows exactly which days the clock was running.
This is the sort of rule used in support terms, repair service levels and some contracts. To count whole days rather than hours, use the Business Days Calculator. To measure the gap between two clock times, use the Time Duration Calculator, and for sums of hours worked see the Hours Calculator.
Matching your own schedule
Set the day to match the hours that count in your agreement. If lunch is not counted, add it as a break. If your team works six days, choose Monday to Saturday. List every day off in the range, because a missing holiday moves the deadline one working day earlier than it should be.
The tool works in clock time with no time zones or daylight saving changes, so keep the start time in the zone your schedule uses. The Time Calculator is useful for sums of hours and minutes.
Assumptions and limits
- Every working day uses the same start, end and optional break times.
- Days off are dates you list yourself. No holidays are built in.
- Time zones, daylight saving changes and part-day closures are not modeled.
- Hours are counted in whole minutes, and the search covers up to about ten years.
Frequently asked questions
What if the start time is after closing?
Counting begins at the next time work opens, which may be the next morning or the next working day. The table shows the first day that actually counts.
Does a deadline exactly at closing time roll over to the next day?
No. If the last working minute ends at closing time, the deadline is shown at closing time on that day, for example 5:00 PM.
How do I handle a lunch break?
Open the advanced options and enter when the break starts and ends. Those minutes are not counted. The break must fall inside the work day.
Can I add hours that cross a month end?
Yes. The tool walks day by day with the real calendar, including different month lengths and leap years, and skips the days off you list.