This Sunday, we will all dutifully set our clocks back one hour for daylight saving time. But why is our day divided into 24 hours, each hour divided into 60 minutes, and each minute into 60 seconds?

First, keep in mind that in the 400,000-odd years that Homo sapiens has been around, most people had no use for exact time. Dawn, sunrise, sunset and night were very important, but no cave person ...