bad-uses-of-a-time-machine
Explanation
The Joke
The comic is titled "Bad Use of a Time Machine #161803: Making a Pun at the Most Offensive Moment Possible." A time traveler appears in a time machine next to Julius Caesar, who is lying on the ground dying from stab wounds (the Ides of March assassination). The time traveler cheerfully asks: "Hey, Caesar! Are you having a knife day? Ehehehehehe."
The Humor
The joke operates on multiple levels. First, the pun itself is terrible: "knife day" instead of "nice day," delivered to a man who has just been stabbed to death -- possibly the worst possible moment and audience for that particular pun. The humor comes from the sheer tastelessness and futility of using time travel technology for something so pointless and offensive.
The numbering of this as bad use #161803 implies there is an impossibly long list of bad time machine uses, and this is just one entry among hundreds of thousands. This suggests someone (or some organization) has been cataloguing terrible uses of time travel, which is itself an absurd concept.
References
Julius Caesar was assassinated on March 15, 44 BC (the Ides of March), famously stabbed by a group of Roman senators including Brutus and Cassius. He was reportedly stabbed 23 times.
The number 161803 may be a reference to the golden ratio (phi), which begins 1.61803... This would be a subtle mathematical Easter egg typical of SMBC'''s nerdy humor.