Approximately -58000 UC
This might just as well be called The Dawn of Time and its significance is that this is the oldest approximately datable event in Discworld history. Referred to in Making Money, this is - approximately - the time when Um was destroyed in a war with the Ice Giants or buried by a volcanic eruption - accounts vary - give or take two thousand years on either side. Professor Flead of Unseen University is pretty definite about this and asserted as much to Miss Adora Belle Dearheart and to Moist von Lipwig when seeking to date the Umnian Golems, the survivors of the catastrophe that destroyed Um, and which had spent nearly sixty thousand years buried deep beneath the surface of the Disc.
Making Money: For what it’s worth, we think there was a natural ice dam up in the mountains, which burst and flooded half the continent. The stories about Um say it was destroyed in a flood, so that fits. The golems were washed away with the rubble, which ended up against some chalk cliffs by the sea.”
“How did you find out they were down there? It’s…well, it’s nowhere!”
“The usual way. One of our golems heard one singing. Imagine that. It’s been underground for sixty thousand years…”