Just like Gmail now you can use Google calender offline specially when you do not have a Internet connection.
Quoting from Gmail Official Blog
"To enable offline Calendar access, sign in to Google Calendar and look for the "Offline Beta" link in the upper right-hand corner of your account, next to your username. We've released this early and are still ironing out some kinks, so if you encounter any issues, be sure to let us know. If you access Calendar through the Premier or Education Editions of Google Apps, your domain administrator will first have to elect to turn on new features from the Domain Settings page of the Google Apps control panel."
