7 Eleven
7 Eleven
[A person in a spacesuit is trying in vain to open the door to a convenience store, rattling the handle. Behind him stands a tall post with a big 7-eleven logo at the top and the opening hours on a bar below the logo.] Sign: 7-Eleven Bar: Open 24 hours Door: Rattle rattle
[Caption below the panel:] I'm glad they finally opened a 7-Eleven here on Mars, but it's annoying how it closes for 37 minutes every day.

This comic pokes fun at the idiosyncrasies of time keeping.  Since units of time are intimately tied to a planet's rotation, and planets rotate at different rates, time keeping doesn't always follow a simple pattern.
Many stores advertise being open 24/7, which means that they're open all day, every day. Many locations of the convenience store chain 7-Eleven are now "open 24 hours", again meaning they are always open (despite historically being open only from 7 AM to 11 PM local time, hence its name).
The main joke in the comic refers to the fact that a day on Mars (the time it takes for Mars to make a full rotation on its own axis) is about 24 hours and 37 minutes.  If a 7-11 store is open for literally 24 hours per Mars day, then it would actually be closed for around 37 minutes each day.  NASA, for its Mars missions, uses a "Mars-hour" that is one twenty-fourth of a Martian day; had the sign implicitly referred to 24 Mars-hours then the store would be open for the entire Mars day.
The duration for the Martian day used by Randall is the Martian sidereal day (how long it takes the non-Sun stars to get to the same position in the sky) of 24 hours, 37 minutes, and 22.663 seconds. However, Mars exploration missions use the Martian solar day (how long it takes the Sun to get to the same position in the sky) or sol of 24 hours, 39 minutes, and 35.244 seconds. Thus in practice, the 7-11 store would be closed for 39 minutes daily instead of 37 minutes. Likewise, Earth time usually refers to solar days; a typical (mean) sidereal Earth day is 23 hours, 56 minutes, 4.0916 seconds long.
The first part of the title text refers to daylight saving time (DST), where clocks are changed on predefined days of the year in order to maximize use of available sunlight.  In the United States, most places set clocks forward by one hour on the second Sunday of March, resulting in a 23-hour day, and back again on the first Sunday of November, resulting in a 25-hour day.  Thus technically, even a 7-11 in the US would not truly be open "24 hours" every day.  Arizona and Hawaii are called out as exceptions because they do not observe daylight saving time (except on the Navajo reservation in Arizona). Randall has made fun of DST several times before , and once again he shows his disdain for DST by saying that in the U.S., only 24-hour stores within the two states not using DST are honest. This comic came out over a month after DST began in the US.
The second part of the title text refers to leap seconds , which may be added or subtracted to the end of June or December in order to synchronize time with Earth's actual rotation.  Months with a leap second will see its last day being one second longer than 24 hours.  Since leap seconds apply to all Earth-based clocks, any store on Earth would not technically be open for exactly 24 hours on such days. Leap seconds have been referred to before in the title text of 1514: PermaCal .
[A person in a spacesuit is trying in vain to open the door to a convenience store, rattling the handle. Behind him stands a tall post with a big 7-eleven logo at the top and the opening hours on a bar below the logo.] Sign: 7-Eleven Bar: Open 24 hours Door: Rattle rattle
[Caption below the panel:] I'm glad they finally opened a 7-Eleven here on Mars, but it's annoying how it closes for 37 minutes every day.
