The shared online calendar is the de facto standard for event organisation and management in the modern office environment. It is also a potentially valuable source of context, provided the calendar event data represent an accurate account of 'real-world' events. However, as we show through a field study, the calendar does not represent reality well as genuine events are hidden by a multitude of reminders and 'placeholders', i.e. events that appear in the calendar but do not occur. We show that the calendar's representation of real events can be significantly improved through data fusion with other sources of context, namely social network and location data. Finally, we discuss some of the issues raised during our field study, their significance and how performance could be farther improved.