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.