The CriticalChange property returns the date and time at which a critical change occurred for an item.
Appointment and Attendee objects
Date/Time
Read/write
Applications can use the CriticalChange property to determine if items are the most current. For example, suppose a meeting request is sent to a number of users, and then later, a revised version of the meeting request is sent that includes a critical change such as a new meeting location. Before the revised version is sent, a user responds to the original version of the meeting request. This property enables the application to determine that the user’s response to the original meeting request is out of date.
This property is used only with meeting requests. Normally, applications will use this property in combination with the ChangeNumber property to determine if critical changes have occurred and when they occurred.