Is there an 'offline' version of Google calendar for Android? For the PC there is a plugin for the google calendar so that edits to the calendar can be made offline. On my Android 2.1 there does not seem such an option. 

I am using the calendar though the browser and not an app. On the app market place I cannot find the app so I am using the browser..
As I said in several comments, the Calendar app is the way to interact with your calendar offline.

If you're interacting with the calendar via the browser, you need to be connected. The Android browser doesn't support any sort of "plug-in".