The "skyblue clocks" on Boylan's socks are not representations of timepieces. In 19th century sartorial language, a clock was a decorative pattern on the sides of socks, sometimes adorning only the outer ankles, sometimes going up both the outsides and insides. At the time represented in the novel they were the thing for fashionable dressers to have.
The OED lists one meaning of the word clock as "An ornamental pattern in silk thread worked on the side of a stocking." It cites examples from as early as the 16th century. The American Heritage Dictionary defines the clock as "An embroidered or woven decoration on the side of a stocking or sock," noting that it was "Perhaps originally 'a bell-shaped ornament,' from Middle Dutch clocke, bell." Webster's adds that the embroidered or woven ornament goes "up from the ankle."
Although it is not quoted in Ulysses, one couplet in Those Lovely Seaside Girls calls attention to the popularity that these decorations were enjoying at the turn of the century: "The boys observe the latest thing in socks; / They learn the time—by looking at the clocks." The joke, of course, is that the boys are studying not the time but the female body. With the lifting of hemlines off the floor in the later Victorian era, socks gave the male eye glimpses of thin cloth covering a woman's legs, with some of the erotic charge associated with underwear. And clocks on those socks would lead the eye in the right direction.