I believe it's been implied, but not directly stated, that Pandora hadn't reset in over 200 years. Then again, maybe I did state it somewhere in the comic itself. There's a lot in those archives, and one line is all it takes.
Pandora's "two hundred and ninety-nine" comment in panel three is inspired by people perpetually claiming to be 29 (as in "younger than thirty") for several years and is so organic in nature that I refuse to believe I'm the first to write it, but I've never seen it anywhere.
Nonetheless, there are some jokes that are so natural that you just know you can't be the first to be saying them even if you've never heard them before, and that one's GOTTA be out there somewhere. That said, I liked it too much to let that stop me.