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She took a factory job working from sun-down to sun-up. (Source: Drifting )

I think sun-down expression seems to have been made from "the sun is down", but what I want to know is what is the rule that I can make such an expression.

For example, if I want to express the time when the moon is up, can I say "moon-up time" ? and what is the reason to add a hyphen ("-")?

Do we have to add the symbol when an expression does not follow grammar so as to prevent readers from misunderstanding?

If you make some examples similar to the expression, it must be a great help for me!

I think this is no different in English than in your own native language. You can make up almost anything you like, but you have to consider the following:

1. Has it been used before, and if so, does it already has a defined meaning?


2. Does it accurately convey the meaning you intend?


3. Will it sound clever or will it sound awkward?


4. Is there another expression that already exists that sounds better?

In this case "moon-up" makes sense, but is unnecessary since we already have the elegant expression moonrise. There's no point in making up a new phrase that doesn't sound as good as the existing phrase.

As for what sounds clever vs. awkward in English, well ... that's just something you have to acquire with experience, plus some degree of skill. As with any language, there are many native English speakers who have no sense of how to be clever -- or, all too often, think themselves clever when they are nothing of the kind.

Meriam-Webster gives actual definitions for both of those words.

Sundown:

: SUNSET sense 2

Sunup:

: SUNRISE

In short, both of these are fine:

• She took a factory job working from sundown to sunup.


• She took a factory job working from sunset to sunrise.

Merriam-Webster also defines similar words as they relate to the moon.

Moonrise:

1 : the rising of the moon above the horizon
2 : the time of the moon's rising

Moonset

1 : the descent of the moon below the horizon
2 : the time of the moon's setting

So, you could also say:

• She took a factory job working from moonrise to moonset.

(This would not be entirely idiomatic, but the syntax and grammar is fine.)

Last, you could express the same thing in the most idiomatic way:

• She took a factory job working from dusk to dawn.

Asking about conventions of punctuation and hyphenation is too broad of a question to answer in this context. Although grammar is related to style, it's not the same thing. When it comes to making things up so as to describe them in ways that haven't been defined yet, it's a matter of personal opinion, based on whatever conventions you follow that would seem to make the most sense to people in general.

This is why there are such things as style guides. But, in this case, I was able to address the specific example sentences by referring to a dictionary.

(And note that the difference between, for instance, sun down, sun-down, and sundown is just a matter of what dictionary you use and what style you follow.)

Last, the etymology of a word or phrase can often only be answered on a case-by-case basis, and, many times, it's simply a matter of speculation why things came to be the way they ended up.

First, the sentence is an idiomatic way to say that she was working a long night shift. Her work hours weren't really tied to the sun's rising and setting. Depending on a city's latitude, the difference between when the sun sets in the summer and winter can be as much as four or five hours. In a poetic sense, the book is saying she arrived to work around sunset and was let off of work as the sun was coming up. That may have been true in the spring and fall, but her hours would not have been adjusted in the winter and summer to account for the later and earlier sunrises. (Maybe on a farm, but not in a factory.)

Second, I wouldn't say that sun-down was made from "the sun is down" – it's more like it was made from "when the sun is going down" (or "almost down"). As another answer says, it's a synonym for sunset, while sun-up is a synonym of sunrise.

Now, about when you can coin such phrases and when you can omit the hyphen: many times a two-word phrase may start with a hyphen, and then, as the expression become more commonplace and idiomatic, the hyphen is eventually dropped. This happened with the word tomorrow in the early 1900s (see the ngram), and it happened with sundown in the early nineteen century. The ngram for sunup is following a similar trend; however, the difference isn't as dramatic, and I suspect there are two reasons for that. First, the word sun-up simply isn't used nearly as often as sun-down (perhaps because we have the word dawn for that). Also, the two u's so close together in a word like sunup looks awkward, while sundown looks like a more conventional compound word, so it's understandable why some writers might want to retain a hyphen in sun-up but be comfortable with dropping it in sundown.

An expression like moon-up is feasible (and is even established), but it's very rare and some readers might even find it a little jarring. Using a phrase like moonup (instead of a more common one like moonset) is always a little risky if don't want to sound awkward, but there are occasions when you it might work. For example, in a science fiction story on a planet with a very large and bright moon, the people might use the word moonup ubiquitously because so much of their life might be effected by the moon's rise.