- 月日の経つのは早いものだ。
- 月日が経つのは早いものだ。
These sentences are syntactically exactly the same. Changing の to が doesn't change the meaning of the sentence a bit. The particles after 月日 are subject markers, and 月日{が/の}経つ is fundamentally a relative clause that modifies の (which is a noun).
Have you wondered why there is something called "ga-no conversion" in modern Japanese? You may believe が is the subject marker and の is the genitive case particle, but this distinction is rather new. In the past, these were almost the same particle. Both could mark a subject, and both could work as a possessive particle.
- Why can の and が both mark subjects in relative clauses?
Old Japanese, as far as we can tell, didn't have a dedicated subject marker - if you wanted a subject that wasn't the topic also, you just left it unmarked. It had two genitive particles, though, *nə and *ŋga (modern の and が); which varied according to a kind of animacy hierarchy - *ŋga with personal pronouns and names IIRC (I've forgotten some of the details), *nə with the rest.
- 「が」vs「の」 with possessives
- Why is there a が in 深淵に臨むが如し?
- 【古典】古文の格助詞「が」「の」(主格・連体修飾格・同格)
Although が and の have become very different in modern Japanese, this is why they are still interchangeable inside "relative clauses".
And the concept of "relative clauses" is a bit broader than the English equivalent. What you know as nominalizers and cleft sentences are actually a special type of relative clauses! Ga-no conversion works also inside these constructions, as shown below (S = subject marker, A = attributive form of a verb/adjective/copula, N = modified noun):
- 太陽がS昇るAのNを見ていた。
太陽のS昇るAのNを見ていた。
I was watching the sun rising. - 彼がS来るAのNは知らなかった。
彼のS来るAのNは知らなかった。
彼がS来るAことNは知らなかった。
彼のS来るAことNは知らなかった。
I didn't know that he would come. - 彼がS知っているAことNを教えてください。
彼のS知っているAことNを教えてください。
Please tell me what he knows. - 彼女がS1読んだA1のN1は表紙がS2赤いA2本N2です。
彼女のS1読んだA1のN1は表紙のS2赤いA2本N2です。
It is a red-covered book that she read. (cleft sentence) - 月日がS1経つA1のN1がS2早いA2ことN2がS3嫌いなA3理由N3がS4分からないA4人N4がS5したA5質問N5
月日のS1経つA1のN1のS2早いA2ことN2のS3嫌いなA3理由N3のS4分からないA4人N4のS5したA5質問N5
a question made by someone who don't understand the reason why some people dislike the fact that time passes quickly
Notice the S-A-N pattern seen consistently in these examples. Don't mix the の as a subject marker (marked with S) and の as a formal noun (marked with N).
So, after looking at these examples, haven't you started to think it's not very meaningful to treat one as a subject marker and the other as a genitive case particle?