There are many ways to shape time into meaningful compartments. Photographers talk about the decisive moment. Pop culture tends to think in decades--the Roaring Twenties, the Swinging Sixties. Some historians are satisfied by nothing less than what French thinkers call the longue duree, the centuries-long unfolding of social and even geological change.
In a faster-moving world, we find ourselves thinking about days and the way that a single one--think D-day, think 9/11--can have consequences that play out for decades. This month marks the 80th anniversary of the day in 1923 when TIME published its first issue. Our magazine began as...