Google has announced its plans to depreciate open standard CalDAV API for calender syn and will replace it with it's own proprietary Google Calendar API.
CalDAV API will become available for whitelisted developers, and will be shut down for other developers on September 16, 2013. Most developers’ use cases are handled well by ,Ic which we recommend using instead. If you’re a developer and the Calendar API won’t work for you, please fill out this form to tell us about your use case and request access to whitelisted-only CalDAV API.
CalDAV specification was authored by Apple, Oracle and CommerceNet and is often criticized by developers for being overly complex.
Open Source advocated are concerned about this move accusing Google of moving away from open standard. Google already doesn't support ISO approved ODF format across it's services and platform including Google Drive, Android and Chrome OS.
Red Hat's Jan Wildeboer posts on Google + "Next: Deprecate IMAP for gmail? It wouldn't surprise me anymore. The world needs better and open standards. Not implementations that aren't interoperable."
Wildeboer futher adds "For me open standards come first. Open Source implementations of open standards will always win in the long run. But interoperability MUST be based on open standards. And in that regard Microsoft and Apple are partly doing a decent job (ODF 1.2 in M$ Office is quite good, Apple supports CalDAV and CardDav and even publish an open source server for it at http://calendarserver.org/ ). For these good efforts I have a lot of respect, for quite some other things I don't. But it simply isn't all black and white."