One reason why the commence date of a consecutive voyage can be incorrect is due do a previous voyage not being synced properly.
To correct this you must roll back the voyage(s) to where the sync broke by setting the status to scheduled. This will automatically clear out port activities. Once setting the voyage statuses to scheduled, proceed to save the last know voyage with the correct completion date. (Ensure all voyages are selected as consecutive voyage). The voyage will now have the correct commence date.