Here's possible fix that seems to be working with my online (Outlook.com) Calendar via the People listings. Delete the "Year" associated with their birthday, then go into Calendar birthday settings and hit the Refresh option. Then the birthdays associated with your contacts might possibly autofill into your calendar. I've always listed years in my contacts' birthdays (when I knew the year, anyway), and my calendar seemed very buggy for birthdays. Then I saw an idea that the syncing problem might be related to having or not having a Year with the birthday. I've now deleted several birthday Years from my contacts and the Calendar now seems to be pulling from People. It's still annoying that it won't work with a Year selected when the Year box is offered as an option in People, but at least it's something of a fix for birthdays auto filling into your calendar like the system is supposed to work.
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