Usage of fugio as an idiom to mean forget

Question

I am confused how fugio is used grammatically when it is used idiomatically to mean forget. In Latin the regular word for forget is dedisco (to unlearn). However, usually the Latins used various idioms to capture this meaning. For example, the word obliviscor (to darken) is used sometimes to mean forget, the idiom being that the "mind darkens". Another word that is used is praetereo (to pass by), the idea being that the mind "passes by" or overlooks something. However, probably the most common idiom used to mean forget was fugio (to flee). So, for example, we have this statement by Macrobius:

Hunc ergo ordinem Romanis quoque imitari placuit, sed frustra, quippe fugit eos deum unum, sicut admonuimus, additum a se Graecum numerum in honorem imparis numeri.

(Therefore, it pleased the Romans to imitate this system, but in vain, because of course they forgot the one day added to the Greek count by themselves in honor of the odd number, as we mentioned above.)

I don't quite understand the grammar of this, specifically where it says fugit eos deum unum. The first part apparently means (it) fled them, meaning they forgot, where eos refers to Romanis. But of what is deum unum the object of? It is in the accusative, so it is clearly meant to be an object, but there is no verb of which it is an object. Is the idea that there is an imaginary word "forget" and deum unum is the object of that imaginary word? In other words, the idiom, fugit eos, could be thought of as being the same as dedidicērunt. So, the analogy is that we can read the phrase instead as:

quippe deum unum dedidicērunt additum a se Graecum numerum ....

So, essentially there are two accusatives in the phrase, eos, which is the object of fugit, and deum unum, which is the object of the entire idiom <fugit eos>. Is that the way to understand it?

Answer 1

According to John Jackson, later editions have an alternate reading:

Hunc ergo ordinem Romanis quoque imitari placuit, sed frustra, quippe fugit eos unum diem, sicut admonuimus, additum esse ad Graecum numerum in honorem imparis numeri.

Jackson notes: "the common and later Editions read esse instead of a se, which latter is evidently the true Reading; and is the Reading of the old Edition A. D. 1524." Whether or not esse was in the original, it can be understood from the context.

Also notice that instead of deum unum, the text used by Jackson as well as many others have unum diem along with the preposition ad.

This is a case of indirect speech, which involves the formula accusative + infinitive. In this case, the accusative is unum diem and the infinitive is esse.

It escaped them [that] one day… was added to the Greek number…

Answer 2

Indeed something fled the Romans, and that something was expressed by accusativus cum infinitivo. The key is to supply the implicit esse explicitly with the past participle:

fugit eos deum unum additum [esse]
it escaped them that a god had been added

Now deum (or diem if we are to read it as "day") is the subject of esse and things start falling to place. The parenthetical remark splits the ACI, making the sentence harder to parse.

The adder is indicated by the agent a se, referring to the Romans themselves. I would have expected addere ad Graecum numerum for "to add to the Greek count", but perhaps the preposition was felt unnecessary as it is already present as a prefix.

The answer by Expeditio Bipes gives an alternative version of the text which indeed has diem and explicit esse and ad. It lacks the agent, but that is easy enough to supply from context.