What does "頃のこと" do in this sentence?


The full sentence, from Anki core 2k/6k:

かつて私が学生だった頃のことです。

"That was when I was still a student."

I'm understanding "かつて私が学生だった" to mean "I was once a student," so I'm guessing
the rest needs to account for "that was when." Is こと just the unnamed _thing_
being referenced with the word "That" at the beginning of the English
translation?

Thanks in advance.




Here's the breakdown:

> (かつて私が学生だった→) **頃**  
>  **time** (←when I was once a student)
>
> の  
>  's
>
> こと  
>  event
>
> ～です。  
>  (It) is ～.

こと refers to an event or occurrence rather than a "thing". 頃 is a noun that
means "time" or "age" (See: [What is the difference between 頃 and
とき?](https://japanese.stackexchange.com/q/28545/5010)), and かつて私が学生だった is
modifying 頃 as a relative clause.

This use of の may seem unfamiliar, but English speakers say " _yesterday's_
event", " _this year's_ sales" and so on, so it should not be difficult to
understand this.

