14 DAY TRIAL // As expected, Google Calendar has gotten an offline option, which has now been enabled for most people. Google announced earlier that Calendar, along with Docs and Gmail, will get some offline mode support.
Out of the three services, Calendar is probably the most usable in offline mode. That said, it's also the simplest of the three and support is far from complete.
Google Chrome users will notice a new Offline menu entry in the Settings menu (the sprocket icon in the right corner). Click on it and users will be prompted to install the Google Calendar app from the Web Store, if they haven't already.
Once installed, users can enable Offline mode. Note that this only works for Chrome users for now, though Google hopes to be able to offer the functionality in other browsers soon.
Google also promised to add more functionality to the existing offline support, but this too will take time.
While offline, Google Calendar users will be able to check out their appointments, i.e. view some of their calendar entries. Older entries may not be listed. They will also be able to RSVP to events created by others.
Crucially, users will not be able to create new Calendar entries while offline or edit existing ones. Google has not said whether this is a technical limitation or a user experience one. The search feature is also disabled.
Likely, it's a combination of both. Offline mode uses some advanced HTML5 features to store data locally to make it accessible even without an internet connection.
Google could enable users to create events as well, also to be stored locally until a connection is available, and will probably do so at a later date, but has decided to not enable the functionality for now.
Worries that an event created never gets synced to the web, potentially leading to people missing appointments, may have been a concern.