VCalendar

vCalendar (VCS) is a calendar format that is a predecessor of iCalendar. It was created by the Internet Mail Consortium (IMC), but the main place it has been used is in Microsoft Outlook, so it is often regarded as an "Outlook format", and frequently presented that way in sites offering calendars for download. Even in 2014, some sites presenting calendars of events still seem to think that the only place anybody would want to export the calendar to is Outlook (iPhones? Android? Google Calendar? iCal? Sunbird? Who needs 'em?), and thus has an "Export to Outlook" button that gives VCS.

Outlook (especially earlier versions) used vCalendar as its main import/export format, and was very slow to gain decent support for the iCalendar format (which was a new, improved variation of vCalendar), though that is apparently present now in current Outlook versions. Meanwhile, just about every other calendar program, service, and device has supported iCalendar as the main import/export format for many years, causing a divide between those who think the "standard" is whatever Microsoft does and those who go by vendor-neutral standards such as RFC documents. Microsoft even takes the official position that any vCalendar or iCalendar data not created using Microsoft APIs is unsupported with regard to interfacing in any way with Microsoft products; it might happen to work, but if it blows up it's the outside developer's fault even if they followed the RFCs perfectly.

Identification
The first line of a vCalendar file has. This does not by itself distinguish this format, since iCalendar files also start with this, but the second line in vCalendar is  while it changes to   in iCalendar.

Specs

 * vCalendar 1.0 spec

Other links

 * Difference between iCalendar (.ics) and the vCalendar (.vcs)
 * Building VCALENDAR content without an Microsoft API is not supported by MS.
 * Developer support limitations for public protocols