Khalil Gibran is talking about "Yesterday" and "Tomorrow" based on "Today". What he focuses on is Today, the present moment. Memory and dream is not real. Those can exists only within this current moment. Yesterday and Tomorrow is just attribute of Today, the present moment. All the spiritual sages points this current moment as truth and says there is no past or future. Those are just concepts. Khalil Gibran is considered as a highly spiritual person. His quotes contains the deep meaning of spirituality.
You need to focus on the present moment as Khalil Gibran suggests in his quote to be real.
You need to focus on the present moment as Khalil Gibran suggests in his quote to be real.