Commentators universally agree that this verse is a direct command to focus on the present. Today has its own set of challenges and troubles ('evil'), and God provides sufficient grace and strength for them. To import potential sorrows from the future is, as Charles Spurgeon notes, a 'superfluity of unbelief.' The practical wisdom is to live in today, handle its business, and trust that when tomorrow arrives, it will bring its own provision for its own needs.