Why Microsoft Bookings offers odd start times like 9:50

What the increment does, and doesn't do#

Microsoft defines the time increment (Booking page → Scheduling policy, 5 minutes to 4 hours, or a custom value) as the interval between appointments. Its example: with a 15-minute increment, a 60-minute appointment can start at 8:00, 8:15, 8:30 and so on. With 60 minutes, "appointments are only available on the hour."

That example assumes clean inputs: hours starting on the hour and a calendar with no odd gaps. In the two Q&A threads behind this page:

• 2025: a user with 30-minute increments was offered 9:50 and 10:20 starts. Their Outlook meetings often ended at odd times like 9:40, and they had Outlook's option to end meetings early turned on. The moderator's explanation was that Bookings reflects actual calendar availability.
• 2026: a user got every start at :29 or :59, despite 30-minute increments, no buffers and a matching time zone. The moderator agreed the configuration looked right and said increments alone don't decide start times. The only further fix offered was resetting staff hours or recreating the service. The thread has no confirmed solution.

Microsoft doesn't publish the exact slot algorithm, so the checks below are based on the documented settings and these reports.

Checks, in order#

1. Outlook events ending at odd times#

With Events on Microsoft 365 calendar affect availability on (the default), a staff member's Busy events shape their free time. A meeting ending at 9:40 can create a 9:40 or 9:50 start, especially with a buffer. Look at the staff calendar on a day with odd slots. If the odd starts line up with meeting end times, that's the cause.

Fixes:

• Turn off Outlook's option to end meetings early for that person, or accept the odd starts on busy days.
• Use a larger increment (60 minutes) for services where clean starts matter most.

2. Buffers#

A Before buffer means the slot needs free time before the start, and the calendar event starts at the buffer. A 10-minute buffer after a meeting ending at 9:40 makes 9:50 the first possible start. If you use a buffer to create gaps, consider the increment-based approach in buffer time without longer meetings.

3. Custom increments that don't divide the hour#

A custom increment of 40 or 50 minutes gives starts like 9:40, 10:20 and 11:00 by design. If you set a service-level policy (Default scheduling policy off), check its increment as well as the page's.

4. Hours that start off the hour#

Check Business hours on Business information, each person's hours under Staff (if Use business hours is unticked), and any service Custom hours or date ranges under Availability options. Business hours are set in 15-minute steps, so 8:45 is easy to pick by accident. Since the 2026 moderator suggested resetting staff hours, re-enter them cleanly even if they look right.

5. Time zone mismatches#

A page shown in a visitor's local time zone that differs from yours by a half or quarter hour will show starts at :30 or :45 past. That's correct behaviour, not a bug. If it's not what you want, see wrong time slots or time zone.

6. Last resort#

If starts are still off (like the :29/:59 case) after checking all of the above, recreate the service with the same settings and test again. Then open a Microsoft support ticket with screenshots of the staff calendar and the booking page for the same day.