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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 "-" ?

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!

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.

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.