Yesterday I made September entry on my iPhone but it does not appear on either my iMac or iPad mini.
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The article below may be able to help you with this issue.
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iCloud: Troubleshooting iCloud Calendar
Nothing contained in the links he suggested help me.
Do you have iCloud enabled?
Yes, every single setting under iCloud is "on".
So next step is to setup on your Mac properly...
I have already checked the settings on my iPhone, iMac, and iPad. All the iCloud settings are on.
Try posting your problem in the Mac forums, since that's where your problem is.
Retrieving data ...