I assumed that once we decided what a second was, that’s what we’re going off of for that 13 billion year assessment.
I guess that’s mostly correct from what I can find online.
From the point of the big bang or creation or whatever, time has been progressing second by second for 13.7 billion years.
With the consistency of time measurement scientists use, it seems like that’s why they use conventional standards of time to measure how long the universe has been around.