Libraries to access iCalendar Info

We need a library of routines that can be used both in a web page and in a stand-alone Windows app which give us access to calendar data in Apple OS/X iCalendar data. A good model for this is the iCloud application that syncs iCloud calendar with a person's personal iCalendar. This libraries should offer routines that enable the calling application to: 1) query a calendar for busy/free time given a date or date and time; 2) query the calendar for planned activities for the queried dates; 3) write an activity to the calendar for the given date and time, either as a one time event or a recurring event.