And for some, it's still down.
At 9:47 a.m. ET, Google reported on its Apps Status Dashboard page that Calendar users were getting a "Server Error" page when trying to access the app from either the desktop or mobile devices.
Google said its engineers were investigating the issue.
At 11:33 a.m., the company reported, "Google Calendar service has already been restored for some users, and we expect a resolution for all users in the near future." The cause for the problem hasn't been reported.
The outage seemed to affect users around the U.S. It's not yet clear how many countries were affected.
Users who were trying to access Calendar during the outage were fed a "Server Error" page.
The problem even seemed to affect Hangouts. Various reports have surfaced of people unable to connect to a Hangout through Calendar.
As usual, users took to Twitter to vent their frustrations, and have a bit of a laugh, at Google's Calendar problems.
"Google calendar is down and i hope they do a post mortem and explain what happened and then i realized THEY CAN'T EVEN SCHEDULE THE MEETING," tweeted @ftrain.
?@rkiker tweeted, "Google Calendar is down. Where am I"