Hi @derek chan ,
I have followed the below link to share the outlook calendar to external user
Due to the link mentioned in your question is not complete, how did you share your calendar(Email Calendar or publish online)?
If you shared the calendar through publish online and you are using Exchange account, your issue may be related to the setting of publish date range, it's suggested to contact your administrator to change it via PowerShell as michev mentioned, for more information, please refer to this article: Set-MailboxCalendarFolder.
Besides, if you shared the calendar through E-Mail calendar, as I know, we could set the data range like below. And according to my tests, other users could view all items in my calendar between this range, so please kindly try to change your setting and share your calendar again to test.
If the issue still exists, in order to perform targeted tests and further analyze it, please provide me with more information about your issues(such as the type of account and Outlook client etc.).
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