The durations don't depend on the bell or the clock, just on the kind of time measured by the new kind of clock, with hours of constant duration, etc.
But one could also have a new kind of Target, to use a specific clock to define your notion of time, which of course would create new issues. Still, that's the way to cast a powerful ritual, such as It's Morning in America...
I found my old note written down on Ars settings that I'd hidden in a notebook, this is what I scribbled a few years back:
Ancient Babylon - tower of Babel, priests of Nergal, Airy Powers, foreign Gods
Ancient Greece - ancient Minoans threatened by a volcanic explosion, or the classic period with well-educated Athenians fighting off threats?
Rome - years of many emperors, plagues, many religions and gods to use.
Arthurian times for a British Isles setting, and be "Rivals to Merlin" or "Apprentices to Merlin" rather than heirs.
Rise of Islam - Arab conquests are under way in the century and a half before the foundation of the Order of Hermes, so here's a chance to see what's left of the Roman world getting pushed out of North Africa and the Levant in a time when Mercurian magic is weakened.
14th century - the great famine 1314-17 and black death make for apocalyptic games and extreme suffering in contrast to the 12th & 13th centuries of most Ars.
1490s - printing press in full use, voyages of discovery, plate armour, Pope Alexander VI
I probably should have written this down more clearly at the time.
I think this game is called "Pendragon". Though the magic system is ... er ... not as thorough as that of Ars Magica, admittedly.
EDIT: Though you could probably grab the Pendragon Grand Campaign and have an excellent saga outline I suspect