"She never sleeps before 11 p.m."
Translation:Ŝi neniam dormas antaŭ la dudek-tria horo.
If we're supposed to use the 24-hour clock, why doesn't the question use it (e.g., before 2300).
Because in standard English one normally uses 12-hour clock. So in English it could be both ways and in Esperanto it couldn't.
It's similar to translating English irrational short scale into languages with logical and historically motivated long scale (like Esperanto, for example): you wouldn't complain on English sentence for using “trillion”, nevertheless in the language you're translating into you'd probably have to use their phonetical equivalent of the french word billion (“billion”).
irrational short scale
With all due respect, the International System of Units/Système international d'unités disagrees with you. There are three orders of magnitude between each prefix, not six. In addition, in engineering notation, you use powers of ten divisible by three.
Since the world of science and engineering has come down squarely on the side of right—and by right, I mean short scale—there is no reason to use the outdated long scale.
Using the powers 10 ³ⁿ doesn't mean advocating for the short scale, but that one recognises it's useful to have a name for those powers. This need can be both satisfied by using the short scale (thousand, million, billion, trillion, quadrillion, etc.) as well as the long scale (thousand, million, milliard, billion, billiard, etc.).
If you want to know why the long scale is superior than the short scale, I recommend you a rant I made some time ago under another Duolingo sentence. :D
Using "la dek-unua nokte" or "la dek-unua ptm" is just as valid and probably better since the 24-hour clock was not specified and is not commonly used by English speakers (at least in the US and UK). The word "horo" is optional.
This crazy am/pm thing is killing me. The 24-hour format is much more easier to use!
kial ĝi estas "antaŭ" sed ne "antaŭe"? ankaŭ, kio estas la malsameco inter ilin?
Eble, la plej bona traduko de "antaŭe" estas "previously" kaj tiu de "antaŭ" estas "before".