Time Machine
Time Machine
[Rob is working at a workbench. Future-Rob appears out of nowhere with a baseball bat.] Future-Rob: Hi, Rob. Rob: Whoa, you're me!
[Future-Rob holding the baseball bat, standing next to Rob.] Future-Rob: You're about to have an idea for a time machine. Rob: I am?
[In a frameless panel, Future-Rob hits Rob over the head with the baseball bat.] WHAM
[Megan approaches Future-Rob working at the workbench, with Rob nowhere to be seen. The bloody baseball bat is stashed behind it.] Megan: Hey, Rob. What's up? Future-Rob: Nothing.
[Caption below the last panel:] This happens somewhere roughly once a month.
This comic's title is very similar to 1203: Time Machines .

Rob is about to discover time traveling , but a future version of him comes back in time and hits him with a baseball bat before he can actually build this time machine.
A common theme in time travel fiction is going back into the past to fix some mistake or stop some tragedy before it happens (see for instance The Terminator movies). In this comic, it is implied that Rob's time traveling turned out to cause a tragedy of some kind, so in order to stop it, Future-Rob must go back in time to stop himself from time traveling in the first place. The last panel supports this by suggesting that at least once a month somebody discovers time travel, but inevitably ends up going back in time to prevent themselves from doing so.
This is a plot point from the 2004 time-travel drama film Primer : one character intends to travel back in time to prevent them from discovering time travel in this way, and another character has already traveled back in time, drugged his earlier self, and taken over the operation to discover time travel before the narrative of the film begins. Primer has a notoriously complicated plot that Randall already has made a jocular attempt at explaining in 657: Movie Narrative Charts . Some more thorough attempts to explain it can be found here and here .  Doubtless, this has also been spoofed in countless other comedic settings.
The blood on the bat suggests that future Rob actually killed past Rob. This is of course a paradox like the grandfather paradox - but there are theories about how it would still be possible - see the link. The obvious paradox is that when Rob dies the future Rob never existed. But also the time travel Future Rob undertakes uses a technique that is now never invented. This was the reason for future Rob's travel.
The title text states that this is why we never see any time travelers since they would have stopped their own past selves from time traveling. After getting rid of their past selves they would then assume their place in the timeline, hence why a friend would suddenly look older: they have aged, just in another timeline before returning to the past.
[Rob is working at a workbench. Future-Rob appears out of nowhere with a baseball bat.] Future-Rob: Hi, Rob. Rob: Whoa, you're me!
[Future-Rob holding the baseball bat, standing next to Rob.] Future-Rob: You're about to have an idea for a time machine. Rob: I am?
[In a frameless panel, Future-Rob hits Rob over the head with the baseball bat.] WHAM
[Megan approaches Future-Rob working at the workbench, with Rob nowhere to be seen. The bloody baseball bat is stashed behind it.] Megan: Hey, Rob. What's up? Future-Rob: Nothing.
[Caption below the last panel:] This happens somewhere roughly once a month.
This comic's title is very similar to 1203: Time Machines .
