Tonight's the night to prowl the manse adjusting clocks for Daylight Saving Time.
The official switch takes place at 2 a.m. Sunday – though, once you twist your watch stem or push the button on digital timepieces, it'll be 3 a.m.
The annual ritual, which ends the first Sunday in November, when we get the hour back, has its holdouts.
Arizona, for one, does not make the shift – although if you're fastidious about changing your watch and you happen to be traveling through the Navajo and Hopi nations that take up the much of the state's northeast, you'll stay busy. The Navajo make the switch; the Hopi, surrounded by the Navajo, do not.
Hawaii also forgoes the change. It occupies a latitude close enough to the equator that the hours of daylight it sees as the seasons change don't vary much.
A northern hemisphere practice
Globally, the use of "summer" time, as many countries call the change, shows a strong north-south divide. The vast majority of countries using it are in the northern hemisphere, while the vast majority of countries south of the equator don't change their clocks during the longer periods of daylight in the austral spring and summer. (Argentina's Daylight Savings Time, though, has proved vexing.)
That makes sense for countries close to the equator, acknowledges John Lowe, an engineer with the National Institute of Standards and Technology's (NIST) time and frequency division at the institutes's lab in Boulder, Colo. But for countries in Africa and South America that that occupy mid latitudes, the reasons for their different customs are less clear, he adds.
Indeed, the reasons given for making the change at all in the 21st century are not very satisfying, he suggests.
"The idea that it saves energy is tough to prove or test or believe in today's modern world, where today's 24/7 activities seem to be the norm," he says. One recent study even concludes that energy costs go up during Daylight Saving Time.
Others add that economically, it appears to be a wash, with some industries and activities benefiting from the change and others claiming it hurts them. Nor is it clear that the gains or losses claimed are economically significant, at least from a national standpoint.
Whose bright idea was this, anyway?
The notion of moving clock hands around to take advantage of more hours of daylight has its roots in the evolution of the concept of standard time, notes Geoff Chester, a spokesman for the US Naval Observatory in Washington, D.C. Where NIST is the nation's civilian timekeeper, the USNO performs the same function for the military. [Editor's note: The original mistakenly placed the naval observatory with the UN.]
Before the advent of railroads, timekeeping was a geographic mess. Greenwich Mean Time was the basic standard for navigation, particularly for determining longitude. But noon corresponded to the time when the sun was most directly overhead. So clocks in Chicago didn't match clocks in Gary, Ind., or Des Moines, Iowa.
Then, in the 1840s, British railroads developed "railroad time" to inject a sense of order into train schedules that were accidents waiting to happen. The same motive drove railroads in the US to follow suit.
"With 50 railroads operating off of dozens of timetables, if you didn't pay attention, you'd have two trains heading for each other on the same track," Mr. Chester explains. But the time zone boundaries tended to run through major rail hubs.
The notion of standard, one-hour time zones was codified in the US in 1918 as offsets measured against Greenwich Mean Time. Today the standard –coordinated universal time (UTC) – is set by atomic clocks rather than the length of one rotation of the Earth.
The shift to daylight time began in Germany during the World War I as a way to ease the pinch of coal shortages and to give people more time to get work done before air raid blackouts took effect each day.
Other countries took up the practice until the war's end. It was reinstated in America during World War II. The US made the change standard practice in 1996. Since then, Congress has twice extended the stretch of calendar subject to Daylight Saving Time. The most recent took effect in 2007.
Sing along with Tevye: 'Tradition!'
So why bother? When all the evidence is weighed, "maybe the strongest arguments are convenience and tradition," Mr. Lowe offers. "I tend to believe that most people would get along fine without this."
Certainly astronauts do. Crew members aboard the International Space Station set their clocks to UTC. It's tough to think about time zones and Daylight Saving Time when your "latitude" varies moment by moment and you see a sunrise and sunset about every 45 minutes.