As you enter tasks, Microsoft Project schedules them to start on the project start date. Of course, every task won’t start immediately. Later on, as you enter additional information about tasks, such as their dependencies on other tasks (for example, hang clocks on the wall only after the paint has dried), Microsoft Project automatically sets more realistic start dates.
To give Microsoft Project the flexibility to schedule realistic start dates, you should enter the task duration only — not the start and finish dates for a task. If you enter the start and finish dates for a task, you restrict the task to starting no earlier than the start date. For example, if you specify a fixed start date for hanging clocks on a wall and the date for painting the walls slips, you and the clocks are going to get awfully messy. If you specify the duration only, you enable Microsoft Project to schedule the clock-hanging flexibly, moving the start date forward or back if the wall-painting occurs later or earlier than planned. Restrict a task to start on a certain date only when it’s absolutely necessary.
For more information about making tasks start or finish on a certain date, see Chapter 6, "Tying Tasks to Specific Dates."