SLA calculator for business hours
When is a reply due if only business hours count? Enter when the message came in, your target and your opening hours. The calculator skips nights, weekends and holidays, and handles time zones and daylight saving time.
Due
Mon 28 Dec 202615:00
Counting 8 hours of business time from Thu 24 Dec 2026 at 15:00, the reply is due on Mon 28 Dec 2026 at 15:00. On the clock, that’s 4 days later.
- Business time counted
- 8 h
- Time on the clock
- 4 days
- Time zone
- Europe/Amsterdam (UTC+1)
Which hours count
- Business hours
- Counted
- Thu 24 Dec2 h15:00 to 17:00
- Fri 25 DecHoliday
- Sat 26 DecHoliday
- Sun 27 DecClosed
- Mon 28 Dec6 h09:00 to 15:00
Due on Mon 28 Dec 2026 at 15:00.
How the calculator counts
A target in business hours only counts time when your team is working. The calculator walks through your opening hours one day at a time, the way you would with a calendar, and does the sums in real hours.
Step by step
- It starts at the moment the message came in. Outside business hours, the clock starts at the next opening.
- It goes through your business hours day by day, skips closed days and holidays, and takes each open stretch off the target.
- Where the target runs out, that’s the due time. In the second mode it adds up the business time between two moments.
- The timeline shows every day in between: the business hours in light, the part that counts in pink.
Shifts past midnight
Set a closing time before the opening time and the shift runs into the next day: 22:00 to 06:00 on a Friday counts until 06:00 on Saturday. A shift belongs to the day it starts, so a holiday on Friday drops Friday night’s shift and keeps the one from Thursday night. 00:00 to 00:00 is a whole day.
Daylight saving time
Business hours are times on the wall clock, and the calculator counts the hours that pass in real time. Office hours from 09:00 to 17:00 stay 8 hours on the day the clocks change. A night shift from 22:00 to 06:00 lasts 7 hours on the night the clocks go forward and 9 on the night they go back, as it does for the person working it.
A time that doesn’t exist, such as 02:30 on the morning the clocks jump forward in Amsterdam, counts from the moment of the jump. A time that happens twice uses the first. The rules for every time zone in the list are built in: in the EU and the UK the clocks change on the last Sunday of March and October, in most of the US and Canada on the second Sunday of March and the first Sunday of November.
Which clock to use
Pick the time zone your business hours are set in, even when the customer lives somewhere else. If your team works across two time zones, use the one your SLA is written in.
Each weekday has one shift, so a lunch break counts as business time. If your policy pauses the clock while you wait for the customer, calculate the parts before and after the pause separately and add them up.
In Flagbell, the inbox calendar shows what comes back each day: reminders, snoozed conversations and GDPR deadlines.
Questions about SLAs
What are business hours in an SLA?
The hours your team has promised to work on customer messages, for example Monday to Friday from 9:00 to 17:00. A target of 4 business hours only counts time inside those hours, so a message that arrives on Friday at 16:00 is due on Monday at 12:00.
Does a message that arrives at night start the clock?
No. The clock starts at the next opening. With business hours from 9:00, a message that arrives on Tuesday at 22:00 with a 2-hour target is due on Wednesday at 11:00.
What happens when the clocks change?
The calculator counts the hours that pass in real time. Office hours stay the same on the wall clock, and a night shift is an hour shorter or longer that night. It knows the daylight saving rules for every time zone in the list.
How do I work out an SLA due time by hand?
Count the business time left today and take it off the target. Then add whole business days, skipping weekends and holidays, until less than a day is left, and add the rest to the next opening time.
Can I use it for support around the clock?
Yes. Choose “Around the clock” and every hour counts, including the extra or missing hour on the nights the clocks change.
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Flagbell is a helpdesk with live chat, a shared inbox and a help center for every brand. It’s in development: join the waitlist to hear when you can use it.