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Better double-check your appointment schedules this Sunday, March 11. That's when daylight saving time will start — three weeks earlier than usual — in most of the U.S. and Canada. In an attempt to save electricity, the U.S. Congress introduced a provision in the Energy Policy Act of 2005 mandating that clocks "spring forward" three weeks earlier, on the second Sunday in March, and "fall back" a week later, on the first Sunday in November. But the energy conservation that extra hour of sunlight is supposed to deliver comes with a cost: computer glitches that some fear could run to Y2K proportions. Companies with BlackBerrys and older computer applications must make manual adjustments or run software "patches" to revise internal clocks, often expensive endeavors.

The change could be bad news for industries with time-sensitive data like travel schedules, bank transactions and stock market purchases. If computers are off by an hour, check receipts could be logged at the wrong time, million-dollar stock trades could be missed and automated equipment in emergency rooms or manufacturing plants could malfunction — to name some worst-case scenarios. There's also the worry of cascading failures. "Changing the time zone in some applications might throw others out of whack," says Ben Kus, senior technology director at BigFix, a computer management firm. Even if fears of Y2K hysteria are overblown, analysts say, many offices will be nettled by out-of-synch e-mail and Microsoft Outlook calendars — in other words, a lot of missed meetings. Rich Kaplan, Microsoft's vice president of customer service says the change will amount to little more than "a nuisance."

But the annoyance comes with a price tag. Jeffrey Hammond, senior analyst at Forrester Research, estimates the daylight saving time (DST) switch will cost the average company $50,000 in time and labor expenses — a conservative figure that doesn't take into account missed airline flights or forgotten appointments. That's a total of $350 million for the 7,000 publicly traded companies in the U.S. "In the aggregate it will probably be worth it, but right now it's an unfair tax on corporate America and even businesses worldwide that I don't think Congress thought about," says Hammond. Since most of Europe ushers in daylight saving on March 25, two weeks later (and most places in Asia, Africa and South America don't change their clocks at all), the U.S. will be out of kilter with the rest of the world for longer this year. Businesses with overseas offices must use the same kind of patch updates on all their systems; otherwise European employees traveling to the U.S. will have PDAs on the wrong schedule.

Such headaches might all be worth it if the adjustment does what it's intended to do: conserve energy. Whether it will, however, is a subject of disagreement. According to an analysis by the American Council for an Energy-Efficient Economy (ACEEE), by 2020 cumulative consumer savings from the extra hour of sunshine will reach $4.4 billion, and the lowered energy use will eliminate the need to build more than three large electric power plants and prevent nearly 10.8 million metric tons of carbon emissions from contributing to global warming. Rep. Edward Markey (D-Mass.), who introduced the DST amendment with Rep. Fred Upton (R-Mich.), says other benefits include reduced crime, fewer traffic fatalities, more recreation time and increased economic activity. "The entire population is active at 6 p.m. versus 6 a.m., and if we all have an extra hour before we turn on the lights, that's a big saving," says David Moulton, Congressman Markey's chief of staff.

But some find these calculations a bit too sunny. Michael Downing, Tufts University professor and author of Spring Forward: The Annual Madness of Daylight Saving Time, says: "Congress has been studying this for 100 years and has yet to come up with reliable energy savings." Daylight saving does affect people's habits: studies from the the last DST extension in 1986 show that we shop, head outside, play sports, fire up the barbecue, and drive more often once daylight saving kicks in. (Conversely, Nielsen ratings for prime-time TV traditionally fall.) But many of these activities, especially increased leisure driving, offset any environmental gains from the energy savings. "Our demand for power, whether through electric lighting, computers or TV, isn't really elastic," says Downing. "We just ask for it at different times." He points out that with the DST extension, Americans living in Grand Rapids, Mich., and farther west in their time zones won't see a sunrise in November until 8:30 a.m. — which means a lit-up house an hour or so longer in the morning, when most families wake up.

Even the Department of Energy (DOE) isn't convinced changing the clocks will make a dent in energy consumption. "The jury on the potential national energy-savings of extending daylight saving time is still out," Craig Stevens, press secretary for the Department of Energy, wrote to TIME in e-mail. "Our preliminary report, based on decades-old information, indicates a very small amount of energy savings." The DOE will keep tabs on energy consumption during the next nine months and calculate exact savings at the end. If it finds that America isn't getting any greener, Congress has an escape hatch: a provision in the Energy Policy Act of 2005 reserves the right to revert to the old schedule for daylight saving time. Says Kaplan of Microsoft, "Then we'll have to make these fixes all over again."

• Kristina Dell
• The new law turning clocks forward earlier could cause major glitches — and it may not even save energy