Had he been sarcastic? Maybe a little self deprecating, but that was a bit par for the course when it came to him.
Diana seemed clever enough to realize that asking why wasn't going to get her any answers -- and he wouldn't brag here, especially not in this instance, but if he couldn't figure out the why and how of it there wasn't anyone else around here who could possibly manage it either.
"It's been about eight months now," he said, because no one needed the details down to the day or second even if Tony was aware of that figure, too. "So. I suggest getting cozy, it might be a long ride."