2.1.20 [RFC5546] Section 3.7.1, Updating DSTART of a Recurrence Instance Changes RECURRENCE-ID

The specification states, "If the 'Organizer' wishes to change the 'DTSTART', the original, unmodified 'DTSTART' value of the instance is used as the value 'RECURRENCE-ID' property, and the new 'DTSTART' and 'DTEND' values reflect the change."

After an instance's start time is changed, Outlook continues to reference that instance with a RECURRENCE-ID set to the original start time. On import, Outlook treats the value of RECURRENCE-ID as the original start time of the instance.