Why the DST Start Is Local, Not One National UTC Time
The U.S. rule says the transition occurs at 2:00 a.m. local time. That means New York reaches its transition before Chicago in UTC terms, Chicago before Denver, and Denver before Los Angeles.
For software, calendars and scheduling systems, use a location-specific IANA identifier such as America/New_York, America/Chicago, America/Denver or America/Los_Angeles. A fixed UTC offset cannot represent both standard and daylight time correctly across the transition.
Does DST Start on the Same Date Worldwide?
No. The March rule described on this page is the U.S. federal schedule. Other countries and regions can use different dates, different local transition times or no daylight saving time at all.
If you are coordinating an international meeting or trip, do not assume the U.S. spring-forward date is global. Compare the actual named locations for the date you care about.