Setting the project's start or end date
If you know when the project is going to start or end, you can enter a date in the Project Information inspector. Then all of the working hours and work schedule exceptions as set up in the calendar view can be applied properly, and real dates appear in the interface. When your project has a start date, the tasks are scheduled from that point on as soon as possible. When your project has an end date, they are scheduled from that date backwards, as late as possible. You can change a task's dates in the Task Schedule Inspector.
If you don't know when the project will start or end, you can mark the start date undetermined in the Project Information inspector. For the purposes of applying the work week, the project is assumed to start on the first day of the week. The normal work week for the project and for the individual resources is respected, but work schedule exceptions such as holidays are ignored.
In a project with an undetermined date, all dates are represented in the format "T day", for the first day of the project, or "T + x", where x is some time duration. For example, "T+2w 1d" means 2 weeks and 1 day after the start of the project.
When you change the start or end date of an existing project from an undetermined date to a real date, the project may shift as the new position of weekends, off-days, and other exceptions are taken into account.