Hyphens, En Dashes, Em Dashes

Q. Dear CMOS: My author uses 9/11 as a shorthand to refer to the September 11 attacks. When this is being used in the phrase “post-9/11 world,” wouldn’t an en dash be more appropriate than a hyphen because post has to bridge 9/11, which is shorthand for two words?

A. The purpose of an en dash in connecting a prefix and an open compound is to prevent the prefix from appearing to go with only the first word of the compound: post-heart surgery diet is less clear than post–heart surgery diet (or post-heart-surgery diet). Because 9/11 is not an open compound, the en dash isn’t necessary for reading comprehension.