I've wondered this myself.
Short answer: It's a murky subject in lore.
We're never shown clocks or the general mechanical prowess necessary for their manufacture, and the technological norm of Tamriel suggests that their presence would be an anomaly (although note that there
are things like printing presses).
However, people use terms like "six o'clock", "The Clockwork City", etc. and specific erudite groups certainly have the mechanical knowledge to construct a clock. One of the http://www.imperial-library.info/content/minutes-lusty-argonian-historical-society even mentions a clock tower.
I think it's something that simply hasn't been thought out yet, but given those suggestions, you could assume that there might be rare and expensive clocks in Tamriel (which fails to explain how commonly "eight o'clock" and the like are used, but... put it to developer oversight or gameplay limitation, i.e. that they use those terms for the players convenience when in lore they'd really be saying "noon" or "dusk" or using a system like the monastic canonical hours)