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OpenERP's support for calendar synchronisation has thus far not been very thorough, and with version 7, support for the caldav modules was removed (see the release notes). However, there are some modules that might provide the functionality you are looking for, either out of the box or with some customization:
While no official commitment has been made, OpenERP appreciates the need for such a feature, and I personally have heard some rumblings of the possibility that work could start on this feature in the future, but don't hold your breath...
The first link points on a page 'google_calendar' which is a module who didn't work or didn't exist. The 2 others links are dead.
This answer is not an answer.
So, actually, this function which is essential, as the function which permits to have our contacts in our address book, isn't support in OpenERP v7.
What could be made is a cron task who synchronizes OpenERP's users, calendars (Meetings, Attendees, Tasks, ...) and contacts (with rights management) with a calDAV/cardDAV server. I fund this project (in Python) which could be a base : Radicale
This is not an answer. But it is a question to all: I have a different question about calendar, did you know,
how to implement Islamic calendar in openerp7.0?
I tried many ways, but no output. Can anyone help me? I go through all question based on calendar in this openerp forum. No result.