Well, it doesn't have to be linear time, does it? Even in the last, say ten years or so of Vimes' lifetime Discworld tech has moved on far more rapidly than the Roundworld equivalent. It doesn't map exactly. The clacks are evidently meant to be the equivalent of email. Then, considering the glued-together nature of Discworld history, it could be the Discworld's genuine 19th century in Borogravia while it's late-20th in Ankh-Morpork.

Yes, Lady Sybil was always in the 1920s or so. Although IMO she'd be more at home in a Christie than a Sayers.