Time puzzled St. Augustine: “I know well enough what it is, provided that nobody asks me; but if I am asked what it is, and try to explain, I am baffled.” Measurements are usually of something distinctively itself – a hundred meters of road, of meadow, of lake – but a hundred hours, happy or sad, is a hundred hours of … time.

Time's insubstantiality defied St. Augustine's understanding and defies ours, but allows humans to press upon it their own conceptions of its parts. It is not odd that medieval Western Europeans took in measuring time their first giant step forward in practical metrology. Nor is it odd that they did so in measuring hours, rather than in calendar reform. Hours were not bounded by natural event, but were arbitrary durations and susceptible to arbitrary definition. Days, in contrast, had such boundaries in darkness and light, and, furthermore, calendars were artifacts of millennia of civilization, stiff with encrustations of custom and sanctity.