If i change data to week, then to 2 day and again and then complite (or not). How can i see when i created my task first ?
It is not possible to view the creation date of a task yet. If you like, feel free to add it in our Votebox https://todoist.com/Vote/.
As for completed tasks, you can view them by clicking on your Karma points. You can then select "View All Completed". Once you can see your completed tasks, you can then filter it by date, project, and label. You can also see the time of completion beside the task.