Perhaps you could refer to a "standard" CalDav client interface included with Windows? Or .Net? I can't think of any OS that provides a standard library for this... Maybe Linux has a few, but not sure that it's in the box... same for OSX.
OSX: iCal
Linux: Evolution/Sunbird/Lightning (Thunderbird plugin)
I'm not sure if Outlook is still shipped with Windows but IIRC I remember Outlook supporting CalDav in the past.
I do realize that all of these run in userspace and technically are not a part of the Operating System but they are provided with the distribution/purchase.