Starting today, Google will let its users add a message whenever they change or delete an existing Calendar meeting.
Whenever a user now needs to change a calendar event, a dialog box will appear, allowing users to enter a message for other guests of the meeting.
Once the message is sent, other guests will see the message in their email alerting them about the updated meeting details.
This is no doubt a useful feature, but hopefully you have a good enough reason for changing a meeting time.
Source: Google Blog