Image text: And every day it gets harder to fight the urge to su to the user and freak people out.

This is a play on words with "Ghost in the/a Shell".  Ghost in a shell is a concept from cyberpunk and especially the Japanese franchise by the name "Ghost in the Shell" that means when a human's brain and consciousness is inside a mechanical body.  Ghost is the person and shell is the mechanical exterior.

In this comic, the play on words is saying if a person dies, they leave a "ghost" of themselves still logged on a server. And of course zShell is a Unix login shell.  Which pulls the whole play on words together.

And the image text references "su to the user". Su is the Unix command to switch to another user from the command line.

Image text: If only we'd chosen 1944-12-02 08:45:52 as the Unix epoch, we could've combined two doomsday scenarios into one and added a really boring scene to that Roland Emmerich movie.

This comic is explained by Ricardo. My name is Ricardo Almeida, I'm a systems analyst from Brazil and a big fan of xkcd, Star Trek (my online persona is Worf), and all things related to science and sci-fi.

if you want to link somewhere here's my Facebook profile page: http://www.facebook.com/profile.php?id=1181078057&v=info

In this comic Randal makes fun of the "Y2K bug" craze and the UNIX 32-bit clock. The UNIX clock "runs out" at exactly 03:14:08 UTC on 19 Jan 2038, with the same potential for "disaster" as the "Y2K bug". As UNIX system engineers are switching to a 64-bit clock, this problem will be averted before it becomes an issue.

In the alt text he makes fun of the prophets of the apocalypse that predict the world ending in 21 Dec 2012 and suggests that if we had chosen 08:45:52 UTC on 02 Dec 1944 as the UNIX epoch (or start time) we could have combined these two apocalyptic scenarios into one, and added a boring scene to the 2009 "2012" movie.

Randal actually made a mistake in his calculations in this one. His epoch time puts the end of the UNIX clock at 12:00:00 UTC on 20 Dec 2012 (midday), a full 12 hours before the December 21. The correct epoch should be 20:45:52 UTC on 02 Dec 1944.