In reality evermoving from immeasurably remote eons to infinitely remote futures in comparison with which the years, threescore and ten, of allotted human life formed a parenthesis of infinitesimal brevity. (17.156)
Bloom is here in the process of showing Stephen out his back door in the middle of the night. The thought itself is no doubt articulated much more clearly by the narrator (whoever that is) than it actually is in Bloom's head. But what about his current situation would get him thinking about the transience of life in such detail? Why here with Stephen? Why so late at night?