The project Start Date and End Date on the Project Details screen is calculated automatically and cannot be set manually.
These dates are calculated based on the tasks which you add to the project. The project start date is the date of the start of the earliest task while the project end date is the date the last task in the project finishes.
To manually "fix" project end date you can add a milestone task with 0 hours duration which starts and ends on specific date.
Please note: At the present moment we do not differentiate between regular tasks and milestone tasks however in the future there will distinct functionality for milestone tasks.