The RecurrenceRule does not have the correct Exceptions when modifying the series. The behavior in Outlook is to clear all exceptions when modifying the series. The behavior in Google Calendar is to consider the exception and modify the recurrence rule accordingly.

Create a recurrence appointment (starts on 20th Feb at 8:00 am, recurring daily), whose recurrence rule will be stored in the database as (Picture 1):

DTSTART:20180220T080000Z
DTEND:20180220T083000Z
RRULE:FREQ=DAILY;INTERVAL=1;BYDAY=MO,TU,WE,TH,FR,SA,SU

Further, I create an exception on 23rd Feb, which changes the start time to 8:15 am. This now, in Addition to inserting a new record for the exception, updates the master's recurrence rule to (Picture 2):

DTSTART:20180220T080000Z
DTEND:20180220T083000Z
RRULE:FREQ=DAILY;INTERVAL=1;BYDAY=MO,TU,WE,TH,FR,SA,SU
EXDATE:20180223T080000Z

So far, so good. Now I edit the master and change its start date to 19th Feb. I expect, that the master keeps the exception for 23rd, but the result is another one. The exception record is kept unchanged as expected! BUT the EXDATE in the master is wrong (Picture 3):

DTSTART:20180219T080000Z
DTEND:20180219T083000Z
RRULE:FREQ=DAILY;INTERVAL=1;BYDAY=MO,TU,WE,TH,FR,SA,SU
EXDATE:20180222T080000Z