I don't see a point in
Day's Undoing for your deck. You have a ton of early creature based mill, which means an early start. The Artifact Combo will only work later in the game and
Dark Deal just offers so much more immediate return that works into your
Dig Through Time to produce nearly the same mill effect of
Day's Undoing without resetting opponent's library.
I use it myself, but only once and only after
Burn Away or at least a
Ghastly Conscription. The exception being versus a creature return, cold
exile or counter spell deck that circumvents creature death.