update waiting time after sharing calendar with external user

Copper Contributor

When I share a calendar with an internal Office 365 user updates to that calendar updates on the calendar appear almost instantly. But when I  share a calendar with an external Office 365 user, there seems to be a long delay before the external Office 365 user can see new events. Can anyone please tell me is this normal? How long should an external user expect to wait before seeing new events?

18 Replies

Hi there, yes, that is the expected behavior right now: sharing internally will sync changes instantly but cross-tenant is still periodic syncing (approximately every 3 hours). I've detailed this behavior in the support article at aka.ms/sharedcalendars, if you scroll down to the section titled "Instant syncing". We're working on releasing instant sync'ing cross tenant but don't yet have a timeline.

Have an open support case with Microsoft on this syncing issue with external users. We have been using this setup for one of our clients and in reality the syncing process has always happened automatically and in a reasonable time frame. Until approx a month ago

Now ics subscribed calendars are syncing intermittently sometimes instantly sometimes outside the three hour windows and sometimes not at all.

This on test events created at the same time with different results.

Now looking at calendar sharing options outside of Microsoft as the service is far too inconsistent for our corporate clients to rely on anymore.

Hi Thyran,


When you create an ICS calendar & share it to someone else (let's say User A), the sync frequency is controlled by User A's calendar service. Unfortunately there's no way to initiate or force a sync from the calendar owner's side.


So if User A uses the URL to add it to an Exchange On-Prem account, the sync frequency is controlled by a setting in the on-prem installation. If User A adds it to an Office365 account, the sync frequency is approximately every 3 hours. If User A adds it to another calendar service, such as Google Calendar, the sync frequency is whatever Google chooses. 


If both users are in different Office 365 organizations, and they share the calendar (rather than publishing via ICS), this is considered "true sharing". Right now, the sync delay is also approximately every 3 hours, but we're working on making it instantly sync for "true sharing", just like we do when both users are in the same organizations.


Feel free to message me if you have more concerns or questions. We care deeply about improving the calendar sharing experience. We've been focused on first addressing all the issues when sharing within the same organization, and then we'll begin working on improving the cross-organization scenarios.

Hi Julia


Thanks for the feedback.


The issue is after testing it for the past three weeks there seems to be an inconsistency with the syncing process for external users cross tenant and third party ics subscriptions

I have been testing this with several of my clients tenants with my work tenant and personal google account.


Events update almost immediately 50% of the time if it is cross tenant. The other 50% of the time half the events are updated almost immediately while the other half don't update at all. This happens even when you delete the events. One event is removed while the other is left behind.


ICS subscriptions with google only show the existing data any new data is not syncing at all regardless of how long you choose to wait. Only removing the ICS subscription and re-adding it shows the new data.


There seems to be a major issue with calendar sharing with external users at the office 365 level at least is problematic at the moment


Have engineers at Microsoft looking into it hopefully get some results soon

@Thyran_Wright I have the same problem.


1. Share the calendar (Main-Calendar "Calendar") with an external person with a normal free outlook-account.

2a) Share via "Sharing" from Web-Version (Office365)

2b) Share via Web (ICS-Link) from Web-Version (Office365)

3. All events will be synchronized the first time.

4. After that, no synchronization for new events. I wait more than 5 hours.


Best regards




Experiencing the same issue. 

First sync with a google account works, but for each  new calendar entry, no updates reflected


Microsoft, can you please provide a solution?

So... "We're working on releasing instant sync'ing cross tenant but don't yet have a timeline." was the reply back in 2017. We're now writing 2021. Could it really be that they're still working on it? At least it seems to me that I'm experiencing exactly this problem still.

@Torben Kirk Wolf Is this still an issue? It seems so, as I am subscribed to an external calendar from Jobber (an online scheduling/CRM platform), and the events are not updated in my calendar until at LEAST 6 hours later, if ever. Microsoft Outlook didn't seem to think this was a problem when I called... Please help us solve this issue! I cannot find anywhere to update the sync/refresh rate either.

Yes, unfortunately, I am still struggling with this. Just today I've accepted two meeting invites that still don't show in my Google calendar :(
Yes, this is still an issue in 2021??? calendar events stop syncing between two O365 tenants calendar sharing approx. 4 weeks ago. tried everything to rectify...even MS support cant figure it out yet. very frustrating to say the least.

We are seeing the same problem, even with synching between personal calendars on Outlook.com and our MS365 tenant - inconsistent and lengthy delays between synchronization.  If instant synch is not working properly even between two Microsoft-controlled calendars, that's frustrating. 


The very slow and inconsistent refresh rate then causes problems when using Microsoft Bookings, which should be respecting free/busy information from a linked Outlook.com personal calendar.  Multiple hours between synchronization makes this impractical.  Please offer a solution, if anyone has it!

I do have the same Issue here, it's really annoying as cross-organization calendar sharing shouldn't be something to worry about nowadays. We often work with external partners / clients where calendar sharing makes life so much easier!

Today (4th November 2021) I am still facing this issue. Sync is so unbelievable slow between two Microsoft 365 tenants with an organization relationship between. Within each of the organizations, it works flawless but across is quite a pain in the **bleep**. I am starting the have a difficulty explaining this to the users... please fit it.

@admin 193whitecrossstreet 


And here we are in 2022 and this problem persists. Sharing schedules is almost useless.

Are there any third party programs that could take care of this sync? this way is absolutely useless!!


Same here. It takes forever for items to show on an external user's calendar