Our command is to concern ourselves with today and let God handle our "tomorrows". It's not that we should neglect our responsibilities of the future, but that we should be casting our worries and concerns onto the Lord. (Philippians 4:6-7)
Don't let what you can't control at the moment rob you of the joy of today.
We are but a mist, and not one of us is promised a tomorrow, so let's seek first the kingdom of God and let Him provide our needs from there (James 4:14 and Matthew 6:33).