Thanks Nick, but still uncertain. When I only enter a duration (for a task with a finish to start relationship as usual) it automatically ticks those boxes. Unticking the start date box removes the duration???
Under what circumstances can you enter a date and not tick the box?
The duration is the difference in working time between the start date and due date, so if there is a duration then both dates must be defined. The checkbox only indicates that a date is present. You can have a start date on its own, a due date on its own, or a start date, due date and duration together.