If a task's Percent Complete field is set to 0, it is considered to be a future task. If set to between 0 and 100, we call it started. If set to 100, we call it done.
If 'Slip Tasks to As-of Date' (in Options, Calculations) is checked, the As-of Date is used to determine the earliest possible date any future task's Start Date or started task's End Date can occur. To enter Start or End dates into a task that are earlier than the As-of Date, it is necessary to update the task's Percent Complete field: when the Percent Complete field of a task is set to between 1 and 99 (inclusive), any date can be entered into the task's Start Date field; when the Percent Complete field of a task is set to 100, any date can be entered into the task's End Date field.