How to change calendar event start time without end time also changing?

When I change the start time of a calendar event, Calendar incorrectly assumes that the duration remains constant so it "helpfully" changes the end time as well. I want to turn off this behaviour. This happens on my iPhone and my iPad. Both are fully up to date, and this is a long-standing bug.

Situation: My work schedule varies from week to week and some days I report to work at 8, some days at 9, but it *always* ends at 6. I create and event on a day, then copy and paste that event onto other days. Say I create an event for 8-6. I then copy and paste that to the following day but on that day I work 9-6. I edit the event so that the start time is 9, but now the end time changes to 7! I then have to manually change it back to 6. Very annoying! In edit mode, start time and end time should be independent, or at least we should be given the option to change start without also changing end. (And I should note that changing the end time does not change the start time, so whatever "logic" was used in the design decision to link them is inconsistent.)