With the date to put on the passport, just pick a date that would be close to when you might be going. Since you aren't thinking about going until next spring, try putting down a date sometime in September/October. It doesn't mean that you HAVE to travel then. They just want something there. If you were applying for it and needed it expedited, then you would have to show PROOF that you needed it that quickly...ask me how I know...
So, don't fret about the date. Just pick at date 6 months from now, that should work.