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Explain xkcd: It's 'cause you're dumb.
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Edge Cake
Every time IERS adds or removes a leap second, they send me a birthday cake out of superstition.
Title text: Every time IERS adds or removes a leap second, they send me a birthday cake out of superstition.


Ambox notice.png This explanation may be incomplete or incorrect: Created by AN EDGY CAKE – Please change this comment when editing this page. Do NOT delete this tag too soon.

Megan (possibly representing IERS) wishes Emily, represented as Hairbun, Happy Birthday. This prompts a confused Cueball to ask if her birthday was sometime last month. Emily explains that she was born over the North Pole in a plane, meaning that she was born in every timezone at once. Technically though this is false, as there are some timezones (such as UTC+5:45) that are not represented at the north pole. Except for the one hour before it's midnight at the International Date Line, the date in eastern time zones is one day ahead of western time zones. She also says that it was February 29th (presumably it was also February 28 or March 1 in some time zones). February 29th only happens once every four years in the Gregorian calendar, adding to the confusion - people born on February 29th often celebrate their non-leap-year birthdays on arbitrary days (or not at all). Normally one could simply use the time in the airline company's native country, but the airline company was changing ownership from one country to another at the time, so this option has apparently been ruled out.

The punchline is that rather than try to identify the correct birthday for Emily, the BIPM has decided to let her have birthdays whenever she wants.

Cueball's final line is a pun on "edge case", an engineering term referring to situations or conditions that are unusual in a way likely to cause problems unless specifically accounted for. Edge pieces are generally only important with sheet goods (brownies sheet cakes, etc), which are typically cut into pieces creating a difference between pieces originating on the edge and pieces originating from the center. Since the sides of a cake are often frosted, an edge piece has two faces covered in frosting and a corner piece has three, while a center piece only has one. Depending upon your relative preferences between the surface (often icing over marzipan) and core body of the cake (which can be fruitcake, or some variety of spongecake, etc, but not actually obvious which until the cake is cut) being an edge-faced slice can be considered a bonus..

The title text states that the IERS sends Emily a cake every time they add or remove a leap second, out of superstition (perhaps Megan is delivering that cake). The International Earth Rotation and Reference Systems Service is in charge of global time standards. It occasionally adds one leap-second to Coordinated Universal Time to adjust for changes in the rotation speed of the Earth.


[Megan is walking towards Cueball and Emily (who resembles Hairbun), holding a cake.]
Megan: Happy birthday, Emily!
Cueball: Wait, wasn't that last month? When's your birthday, anyways?
Emily: It's complicated.
[A diagram of a flight path over the North Pole, with meridian lines radiating out from the center. Emily's dialogue appears above the diagram, but she herself does not appear in this panel.]
Emily: My mom went into labor on an arctic international flight that diverted directly over the North Pole.
Emily: I was born in every time zone at once.
[With Megan standing behind her, Emily holds out a plate of cake to Cueball.]
Emily: It was also February 29th, and the airline was just changing ownership between countries.
Emily: The International Bureau of Weights and Measures finally issued a declaration that it's my birthday whenever I want.
Emily: Cake?
Cueball: Nice, it's all edge pieces.

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