Daylight Saving Time is defined in the US Code. But it has not been updated to reflect the change made in the Energy Policy Act of 2005.

Historical Daylight Saving Time:
First Sunday in April - Last Sunday of October

As amended by The Energy Policy Act of 2005 (begins 2007):
Second Sunday of March - First Sunday of November

The effect of the law is to give us 238 days of DST. Depending on the year, the old rules gave us either 203 or 210 days of DST. The Energy Policy Act adds either 4 or 5 weeks to DST (theoretically saving energy since we'll be up earlier in the day and go to bed earlier at night).

The following Excel formulas calcuate the start and end dates of Daylight Saving Time, following the new rules:
=DATE(Year,3,MOD(1-WEEKDAY(DATE(Year,3,1)),7)+8)
=DATE(Year,11,MOD(1-WEEKDAY(DATE(Year,11,1)),7)+1)

The change may NOT be permanent.

Quoted: ENERGY POLICY ACT OF 2005, SEC. 110. DAYLIGHT SAVINGS.
(a) AMENDMENT.—Section 3(a) of the Uniform Time Act of 1966(15 U.S.C. 260a(a)) is amended—
(1) by striking ‘‘first Sunday of April’’ and inserting ‘‘second Sunday of March’’; and
(2) by striking ‘‘last Sunday of October’’ and inserting ‘‘first Sunday of November’’.
(b) EFFECTIVE DATE.—Subsection (a) shall take effect 1 year after the date of enactment of this Act or March 1, 2007, whichever is later.
(c) REPORT TO CONGRESS.—Not later than 9 months after the effective date stated in subsection (b), the Secretary shall report to Congress on the impact of this section on energy consumption in the United States.
(d) RIGHT TO REVERT.—Congress retains the right to revert the Daylight Saving Time back to the 2005 time schedules once the Department study is complete.