After the official announce of Gmail offline now Google has announced the Google Calenders.In case of no internet or poor quality internet we can read our Google Calendar events but can't add or edit them. Firstly this feature was launched for school and business versions but now off line Google calendar is available for all of us.
To start with 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.
For more details Please Visit Official Gmail Blog: View Google Calendar offline