Is "of" obligatory when writing "In August of this year"?

I saw there was already some discussion on here about "in this year" vs. "this year." I have a similar question, but this time regarding the use of the word "of," and the answers to the previous question don’t seem to address this case.

The sentence I’m grappling with to a stupid degree is: "In August this year, the Belarusian poet published an op-ed…"

I believe we need the preposition "of" before "this year" to indicate possession, no? "In August of this year" reads more naturally to me. But now I’m going crazy trying to find a rule that says using the preposition is really necessary in this kind of construction. Is there such a rule that would provide guidance for this example?

I’m also wondering if the construction sans "of" is more idiomatic in British English…

Many thanks for any help anyone can provide!