Cell phone companies could give you an extra hour of free weekend calls, and people who depend on online calendars may find themselves late for appointments.
An energy bill President Bush is to sign Monday would start daylight time three weeks earlier and end it a week later as an energy-saving measure.
And that has technologists worried about software and gadgets that now compensate for daylight time based on a schedule unchanged since 1987.
"It is unfortunately going to add a little bit of complexity to consumers," said Reid Sullivan, vice president of the entertainment group at Panasonic Consumer Electronics Co. "In some cases, depending on the product, they may have to manually increase or decrease the time."
The upcoming transition evokes memories of Y2K, the Year 2000 rollover that forced programmers to adjust software and other systems that, relying on two digits for the year, never took the 21st century into account.
"It wouldn't be a society-wide catastrophe, but there would be a problem if nothing's done about it or we try to move too quickly," said Dave Thewlis, executive director of a group that promotes standards for calendar software.
Newer VCRs and DVD recorders have built-in calendars to automatically adjust for daylight time. Users would have to override them, switching to "manual" to ensure shows continue to record correctly.
Computers with Microsoft Corp.'s Windows operating systems would need to obtain updates. Though most affected applications would likely be taken care of by the Microsoft fix, calendar systems will need to be checked to ensure that appointments already entered get properly adjusted.
Some electric utilities have advanced meters to adjust rates based on peak and non-peak hours, and studies would be required to determine if any modifications are needed. The telecommunications industry, meanwhile, must ensure that its clocks are properly adjusted to bill customers properly.