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.