[Planner] How is the end date generated or calculated

[Swaroop Shere <swarooponmoon yahoo co uk>]

Hello,
         I am a newbie trying to use the planner software for the first 
time. My problem is how to generate the end date of any task? I read the 
documentation and it said that the software generates end date from the 
effort and resources, but it did not work. And I cannot even change the 
end date manually, as the start date. I am using planner version 0.12.1.
Thank you in advance.
regards,
Swaroop