Sadly, Google Calendar only allows for read-only access while you are offline, which is quite a disappointment.

Gmail, on the other hand, features a 'flaky connection' mode which gives you the option to compose new messages offline. Gmail then sends them automatically once you are back on the Internet. Without being able to enter new appointments or tasks into your calendar, or the ability to move appointments around, Google Calendar's offline feature is not half as useful as it could have been.

Earlier this year, we saw some leaked screenshots that showed the ability to edit your calendar in offline mode, but Google apparently did not make this available yet. This is quite a disappointment, because the 'flaky connection' mode works very well in Gmail. We can only assume that Google will add a more fully featured offline mode in the near future.