Calvin and Hobbes by Bill Watterson for April 22, 2015
Calvin: "Ah! I got the letter I wrote to myself!" Hobbes: "What did you write?" Calvin: "Dear Calvin, Hi! I'm writing this on Monday. What day is it now? How are things going? Your pal, Calvin" Calvin: "My past self is corresponding with my future self" Hobbes: "Too bad you can't write back"