If you have an event and change "dtstart", the end time of the event;
- should not be changed if you specify 'dtend'
- should shift if you specify "duration"
RFC5545 says
; either 'dtend' or 'duration' may appear in ; a 'eventprop', but 'dtend' and 'duration' ; MUST NOT occur in the same 'eventprop'
source share