Prior to 1986, Daylight Saving Time
across the U.S.A. was inconsistent, starting and ending on various dates in different states, sometimes arbitrarily, according to the whims of the current governors. So, when and where it was used, it was not used uniformly. These often variable inconsistencies caused some interstate and international trade, commerce, recordkeeping, communications, travel-scheduling and other problems, so in 1986 a federal law was enacted to regulate its optional usage according to a consistent uniform schedule.
The new law mandated that all states opting
to use Daylight Saving Time, do so uniformly
, starting and ending on the same dates and those dates would be from the first Sunday in April
to the last Sunday in October
. This law applied to the period from 1986 to 2006. Under those rules, the duration of daylight saving time was sometimes 210 days (30 weeks) in some years and 203 days (29 weeks) in others.
In 2007, an amended law extended the duration of Daylight Saving Time from the second Sunday in March to the first Sunday in November
, starting two weeks earlier and ending two weeks later than before, adding four weeks or one more month of daylight saving time to the year than previously. This is the rule for Daylight Saving Time currently in use in the United States. Under this new rule, the duration of daylight saving time is always constant at 238 days (34 weeks).
On the date when Daylight Saving Time starts, it begins at 2:00 AM local time in each time-zone, at which time, the clocks are set ahead exactly one hour so that, at that moment, it becomes 3:00 AM instead. The normally scheduled hour from 2:00 AM to 3:00 AM is skipped over or dropped so, in effect, one hour is lost from the clock that day and the next sunrise and sunset are suddenly one hour earlier than the day before. It gets daylight/dark one hour earlier than previously. Daylight saving time is now in effect.
On the date that Daylight Saving Time ends and we revert back to standard time, it begins at 2:00 AM local time in each time-zone, at which time the clocks are set back exactly one hour so that, at that moment, it becomes 1:00 AM - again. The hour from 1:00 AM to 2:00 AM is repeated. In effect, one extra hour is added to the clock that day and the next sunrise and sunset are suddenly one hour later than the day before. It gets daylight/dark one hour later than previously. Standard time is now back in effect.
With 2:00 AM being used as the transition time, there is never any confusion about the date after the transition either way since the date cannot change.
Public Law 109-58 - 109th Congress of the United States of America