Date
Explanation
The Joke
The comic is structured in two panels with a classic setup-and-reveal format. In the top panel, a man is angrily confronting his date, saying: "Oh, what, you enjoy my company all day long and you're suddenly going to decide it's no second date because I had a single misstep? That's called shallowness, Dave. SHALLOWNESS." The framing makes it seem like the woman is being unfairly judged for some trivial social faux pas.
The bottom panel, labeled "Moments ago..." then reveals what the "single misstep" actually was: the woman cheerfully saying "I've always hated goodbyes, so HEIL HITLER!" complete with a Nazi salute. The joke is the enormous gap between how she characterizes her behavior (a minor misstep) and what she actually did (a Nazi salute as a farewell gesture). What she frames as "shallowness" on Dave's part is actually a perfectly reasonable reaction to an enormously inappropriate gesture.
The Humor
The comic uses the classic misdirection-and-reveal structure that SMBC employs frequently. The first panel is designed to make the reader sympathize with the speaker -- we are primed to think Dave is being unreasonable. The reveal completely recontextualizes the scene, and the humor comes from the whiplash of realizing that Dave's reaction was, if anything, restrained. The comedy also lies in the woman's complete lack of self-awareness: she genuinely seems to think that a Nazi salute is just a quirky alternative to saying goodbye, on par with an awkward handshake or mispronouncing someone's name. Her indignation at being judged for it makes the absurdity even funnier.
References
The "Heil Hitler" salute is, of course, the infamous Nazi salute associated with Adolf Hitler and the Third Reich. Using it casually as a replacement for "goodbye" represents one of the most extreme possible social missteps in Western culture, which is precisely the point of the joke -- the gap between "single misstep" and "Nazi salute" is as wide as it could possibly be.