This is for tasks that have date ranges scheduled.
When you click on the “due date” field and hover over the calendar, it lets you click and select a new date. However, this changes the start date and not the end date.
This is unexpected, unintuituve and unhelpful behaviour. Normally, if you only want to change one date on a task, you would want to extend the end date on a task (especially ones already in progress), not change only the start date. If you need to change the start date, this is less common and you would be selecting it specifically, and probably also changing the end date as well.