It is meant as a guide and criterion for finding what matters the most in one's life and one's existence and organize their way of life around that. If it was y

It is meant as a guide and criterion for finding what matters the most in one's life and one's existence and organize their way of life around that. If it was your last day (and you were a philosopher) you would only concern yourself with the most fundamental and essential aspects of existence and questions of life. It is a reference to the Socratic question and quest: "what is the best course in life". He is instructing people to organize the totality of their lives, in answering and as answer to this question. Even if you do long term planning (and he is not saying you shouldn't) he is telling you to focus and concern that worthy life plan of yours, in answering and as answer to that question. Implicitly, he is also saying that what drives people to waste their time and concern themselves with petty issues, unimportant to their fundamental existence and be drowned in "everydayness", is the ever-lurking-in-the-background assumption that "I have got plenty of time".