If a Task is set up as 'Must Start on...' the end-user is able to start working on the task prior to the start date. Why are they able to start early? The Must Start On date doesn't mean that the assignee can't start working on this before the mentioned date. It just means that you are PLANNING to have it start on that date. There is an Actual dates field that displays the actual start date. The actual date is set by going to 'Setup' > 'Project Preferences' > 'Tasks & Issues Preferences.'