Charles (Charlie) Verboom posted on Sep 28, 2015 
We continue to get reports of issues with the built in calendar app for iPhones: mysterious notifications to people who are not even guests in an event, syncing issues (events on the web that may be updated or cancelled don’t show up on the iPhone) are two examples. Our advice is to use the new Google app – it works and solves many of the issues we have seen over the past several years.
Reference our original article from March of this year
You can get the new Google Calendar app for IOS (and Android devices) here.
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