A task's start and finish dates change when you adjust the duration.
If you enter a task's duration, Project calculates when the task should start and finish based on the project's start date and the duration, as well as any nonworking time, such as weekends. If you later change the task's duration, Project recalculates the dates to correspond to the duration.
For example, if you enter a duration of three days, and the project's start date is March 1, Project calculates the task's start date as March 1 and the finish date as March 3 (unless those dates happen to fall on a weekend or other nonworking time). If you change the duration to four days, the finish date is recalculated to March 4.
If you want to enter specific start and finish dates for a task, enter the task's start and finish dates and Project recalculates the duration. Be aware that entering the start and finish dates sets a date constraint on the task, which limits the flexibility of your schedule. If you enter a start date and then enter a finish date, a Finish No Earlier Than (FNET) constraint is set. Or if you enter a finish date and then enter a start date, a Start No Earlier Than (SNET) constraint is set.
If you want a task to start or finish on a certain date but want to retain the flexibility of the schedule while being alerted if a task will not start or finish on time, you can enter a deadline date for the task. Deadlines do not constrain tasks nor do they affect constraints. As the schedule is updated, an indicator appears if a task is pushed beyond its deadline.