Your project's start and finish dates, and whether it's planned forward from the start or backward from the finish, are set in Project Settings.
Go to File > Settings, or press F8.
Stay on the General tab.
Set a past start date
Tasks can't have dates before the project's start date. To enter past dates on tasks, for example when you're adding work that's already done:
Change Est Start Date to a date on or before the earliest task.
Click Save.
Plan from the start or the finish
Under Plan From, choose:
Start Date: tasks are scheduled forward from the project start. Use this when you know when work begins.
End Date: tasks are scheduled backward from the project finish. Use this when you have a fixed completion date and want to know when work must start.
Click Save after you change it.
Set a deadline
Enter a date in Deadline to record a hard completion date for the project, separate from the estimated end date.
For the other settings on this tab and the rest of the Settings window, see Project Settings Overview.