Learn English – Use of Article before Nation name

One could say:

The students in Mr Smith's art class were at their easels, looking at the sunset.

In that case, the sunset is the object of scrutiny, and it refers to the entire western sky as it is lit up by the setting sun.

But when it is used to indicate a time of day, the idiom is "before sunset", "after sunset", "at sunset".

EDIT: CopperKettle's examples on the use of "the" with a proper noun are good ones. Here are some others in the same vein; perhaps we can extract the essence from them to show when it is appropriate to use "the" with the proper noun.

I do not know you any longer! What has happened to the gentle Henry Jekyll, the Henry Jekyll with the wry sense of humor and a fondness for good port?

Get up off your ass, Jones, and get back on that horse! Where's the do-or-die Jones, the gung-ho Jones, the let-me-at-them Jones who volunteered for this mission??? I don't like this new sissy Jones who is afraid to ride on a pony just because it's a little skittish.

In English, when you don't have a particular place or thing in mind, you have to tell us that with a word. Leaving out a word means something different.

The "zero article" is a grammatical concept in the English language which transforms your singular-form noun into a proper noun (the name of something specific), or a generic mass noun, a little like making it into an abstract concept instead of a real thing or place*. What is the "zero article"? It's simply not having any determiner placed before the singular form noun.

So, unless you are talking about a sleazy bar named "Office" or the abstract platonic ideal of "office" (which is nonsense), we need a determiner of some sort. For example:

• an article (a, the)
• a demonstrative (this, that, those)
• a pronoun (my, his, her, their)
• a quantifier (many, a lot of, any, a few... although these would require you to pluralize "office")
• a number (one, two, three, etc... pluralizing "office" for all but "one")
• a distributive (each, every, either)
• a difference word (another)

If you make use of ANY of the above determiners, you will avoid the zero article. Otherwise, it's a proper name, mass noun, or abstract concept, which is typically nonsense.

Since you want to say that you aren't going to any particular office, you need either the indefinite article: "an", or you need a quantifier: "any".

I don't go to an office. I don't go to any office(s).

Either will work adequately to convey your intended meaning.

* Footnote: There are a limited number of specific exceptions to this rule. "School", "Prison/Jail", and if you are British, "Hospital".