A task's start (start date: The date when a task is scheduled to begin. This date is based on the duration, calendars, and constraints of predecessor and successor tasks. A task's start date is also based on its own calendars and constraints.) and finish (finish date: The date that a task is scheduled to be completed. This date is based on the task's start date, duration, calendars, predecessor dates, task dependencies, and constraints.) dates change when you adjust the duration (duration: The total span of active working time that is required to complete a task. This is generally the amount of working time from the start to finish of a task, as defined by the project and resource calendar.).
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 (nonworking time: Hours or days designated in a resource or project calendar when Project should not schedule tasks because work is not done. Nonworking time can include lunch breaks, weekends, and holidays, for example.), 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 (constraint: A restriction set on the start or finish date of a task. You can specify that a task must start on or finish no later than a particular date. Constraints can be flexible [not tied to a specific date] or inflexible [tied to a specific date].) 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 (deadline: A target date indicating when you want a task to be completed. If the deadline date passes and the task is not completed, Project displays an indicator.) date for the task. Deadlines do not constrain tasks nor do they affect constraints. As the schedule is updated, an indicator (indicators: Small icons representing information for a task or resource that are displayed in the Indicators field. The Indicators field is located to the right of the ID field and appears in a number of tables.) appears if a task is pushed beyond its deadline.