Time travel. Bleh. At least at some point, Mustrum Ridcully, the UU's pragmatic Archchancellor, sets us all straight, at last:

"I can't help thinking, thought, that we may have...tinkered with the past, Archchancellor," said teh Senior Wrangler.
"I don't see how," said Ridcully. "After all, the past happened before we got here."
"Ye [sic], but now we're here, we've changed it."
"Then we changed it before."
And that, they felt, pretty well summed it up. It is very easy to get ridiculously confused about the tenses of time travel, but most things can be resolved by a sufficiently large ego.

In the end, I just didn't love this book. It's odd because the Wizards are some of my favorite characters. Oh, who am I kidding? I love all the characters except Mort and Carrot. It was all the pseudo-Australian bits that grated on me. They just weren't catching me. C'est la vie and onto Carpe Jugulum

As a side note, this is the second Discworld book I've read on the iPad, and I'm so peeved with HarperCollins' digital editions crew that I'm writing letters. They have made a shoddy edition, with typos, incorrect formatting and tedious endnotes. For shame.