Why Microsoft Bookings offers odd start times like 9:50

What we saw when testing (September 2026)#

We captured the public Microsoft booking page and the calls it makes: 13 live captures covering 913 days and 7,924 slots, viewed from several time zones. We then reproduced every slot the page showed from the availability data. This is what the shared page does:

• Slots start where the free block starts. A free block beginning at 6:40 produced 6:40, 7:10 and so on, with a 30-minute increment.
• Each block is walked in fixed steps. The step is the largest of 60, 30, 15, 10, 5, 2 or 1 minutes that divides the increment. A 30-minute increment steps by 30, a 45-minute increment by 15, and a 20-minute increment by 10.
• Then the list is thinned. The page keeps the first start and then any start at least one increment after the last one it kept. That's why a list like 5:00, 5:30, 6:30 appears: 6:00 to 6:30 was busy, so a new block started at 6:30.
• A Before buffer moves the start. In the page code, the Before buffer is added to the beginning of each free block, so a 10-minute Before buffer with hours starting at 9:00 gives a first start of 9:10. (We read this from the code. None of our test services had buffers, so we didn't see it live.)
• Personal "Bookings with me" pages work differently. There, starts sit on a grid of the increment counted from midnight, shifted by the Before buffer, so odd starts are mainly a shared-page issue. See Bookings with me buffers and increments.

The free blocks come from the Microsoft server with working hours, Outlook busy time and existing bookings already applied. Anything that makes a block begin at an odd minute shows up as an odd start.

What Microsoft documents, and the two threads#

Microsoft defines the increment (Booking page → Scheduling policy, 5 minutes to 4 hours, or a custom value) as the interval between appointments, with the example that a 15-minute increment gives 8:00, 8:15, 8:30. That example assumes a block starting on the hour.

• 2025: a user with 30-minute increments was offered 9:50 and 10:20. Their Outlook meetings often ended at odd times like 9:40, and the Outlook option to end meetings early was on. The moderator said Bookings reflects actual calendar availability, which matches what we saw.
• 2026: every start was at :29 or :59, with 30-minute increments, no buffers and a matching time zone. The thread has no confirmed fix. Given how the page walks blocks, a pattern like that points to free blocks that begin at :29 or :59 in the availability Microsoft returns, for example events or hours ending a minute early, rather than to the increment.

Checks, in order#

1. Outlook events ending at odd times#

With Events on Microsoft 365 calendar affect availability on (the default), each Busy event splits the day into blocks. A meeting ending at 9:40 starts a new block at 9:40. 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 the Outlook option to end meetings early for that person.
• Schedule internal meetings to end on the hour or half hour. A bigger increment doesn't help here, because each block still starts at the minute the meeting ends.
• Accept odd starts on busy days. They're real free time.

2. Buffers#

A Before buffer shifts every block's first start by the buffer length. A 10-minute Before buffer after a meeting ending at 9:40 makes 9:50 the first possible start. If the goal is a gap between customers, the increment-based approach in buffer time without longer meetings avoids this.

3. Increments that don't divide the hour#

A custom increment of 40 or 50 minutes gives starts like 9:40 and 10:20 by design. A 45-minute increment is walked in 15-minute steps, so after an odd block edge the next start can land on any quarter hour. If a service has its own 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. Re-enter them cleanly even if they look right, as the 2026 moderator suggested.

5. Time zone offsets#

The page shows slots in the visitor's time zone unless it's set to use the business time zone. A visitor whose zone differs from yours by a half or quarter hour sees starts at :30 or :45 past. That's correct behaviour. See wrong time slots or time zone.

6. Last resort#

If starts are still off after 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. If a day shows no times at all, the checklist in no available times is the better starting point.

BookingsXP is independent and not affiliated with or endorsed by Microsoft.