There doesn’t appear to be any mention of the downtime on Google’s App dashboard but for many the web app has been unavailable for some hours.
There’s no doubting the effectiveness of Google’s tools when available, but with frequent downtime and errors (mainly when attempting to visit their services online), you can understand why some larger businesses who live and die by their calendar are hesitant to commit.
With this particular issue, it appears to not be domain specific. There are users all using Google Apps under the same domain, many with access to their calendar and others without.
We’ve contacted Google for a response, will keep you updated – in the mean time, you can join users in the same boat here.
Everything now appears to be fixed according to this Tweet from the official Google Calendar account.