Quick answer: In the United States, the familiar 2 a.m. changeover provides a local-time transition during a historically quieter period of the night. It is a convention, not a physical necessity.
What happens in spring
The clock jumps directly from 1:59:59 a.m. to 3:00:00 a.m. in participating areas.
What happens in fall
The clock repeats the 1 a.m. hour, which can make time stamps ambiguous without an offset.
Why this hour?
Historically, overnight changes limited disruption to ordinary daytime schedules. Modern systems still have to handle missing or repeated local times.
Are all places the same?
No. Time-change rules differ by country and region, and some never change clocks.
For appointments
When coordinating across time zones, include the time-zone abbreviation or UTC offset and confirm dates around transitions.
The bottom line
In the United States, the familiar 2 a.m. changeover provides a local-time transition during a historically quieter period of the night. It is a convention, not a physical necessity.
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