Google Calendar is now back and fully functional as of 1:02pm.
Currently Google Calendar is showing a 404 error or asking for codes. DO NOT enter any information at this time. Google is currently looking into the issue and we'll update once there's been more developments.
You can also check this link for more updates
6/18/19, 12:13 PM
The problem with Google Calendar should be resolved. We apologize for the inconvenience and thank you for your patience and continued support. Please rest assured that system reliability is a top priority at Google, and we are making continuous improvements to make our systems better.
6/18/19, 11:40 AM
We expect to resolve the problem affecting a majority of users of Google Calendar at 6/18/19, 12:40 PM. Please note that this time frame is an estimate and may change. The affected users are unable to access Google Calendar.
6/18/19, 10:40 AM
Our team is continuing to investigate this issue. We will provide an update by 6/18/19, 11:40 AM with more information about this problem. Thank you for your patience. The affected users are unable to access Google Calendar.
6/18/19, 9:22 AM
We're investigating reports of an issue with Google Calendar. We will provide more information shortly. The affected users are unable to access Google Calendar.