Usually, it is very difficult to quit something "cold turkey" or at once. Many times quitting is not even the goal, but moderation. This way you will learn about the addiction/habit more, and quitting is safe for the body (and sometimes also the mind).
If you set an end date to quit completely there is no hurry or pressure on quitting - and you are ready to say "bye-bye" to the negative behavior when the date comes. Before quitting one should try to break the routine as much as possible.
If you want - and can - quit something all at once, go for it! You can always modify (edit) the goal title and all details. Just remember to find something positive to do to replace your old behavior. Also, try to formulate your goal in a positive manner.