What Is Daylight Saving Time? History, Rules & Impact on Teams

What Is Daylight Saving Time?

Daylight Saving Time (DST) is the practice of advancing clocks by one hour during warmer months so that evenings have more daylight and mornings have less. In spring, clocks "spring forward" by one hour -- typically moving from 2:00 AM to 3:00 AM. In autumn, they "fall back" by one hour, repeating the 1:00 AM to 2:00 AM period.

The shift usually happens on a Sunday in the early morning hours to minimize disruption. In the United States, DST begins on the second Sunday of March and ends on the first Sunday of November. In Europe, it starts on the last Sunday of March and ends on the last Sunday of October. Other countries that observe DST follow their own schedules, and some have abolished the practice entirely.

The net effect is that UTC offsets change during transitions. For example, US Eastern Time shifts from UTC-5 (EST) to UTC-4 (EDT) in spring, while Central European Time shifts from UTC+1 (CET) to UTC+2 (CEST). These changes happen on different dates in different regions, creating a brief period each year where time differences between zones are not what teams expect.

A Brief History of DST

The idea of adjusting clocks to better align waking hours with daylight dates back to at least 1895, when New Zealand entomologist George Hudson proposed a two-hour shift. Benjamin Franklin is often credited with the concept in his 1784 satirical essay about Parisians wasting morning sunlight, though he never proposed changing clocks.

DST was first implemented nationally during World War I. Germany and Austria-Hungary adopted it on April 30, 1916, to conserve coal for the war effort. The United Kingdom, France, and the United States followed within the next two years. After the war, most countries abandoned DST, only to bring it back during World War II.

In the United States, the Uniform Time Act of 1966 standardized DST dates across states (though states could opt out). The Energy Policy Act of 2005 extended DST to its current duration: from the second Sunday in March to the first Sunday in November. The EU harmonized summer-time rules in 1996, setting dates as the last Sundays of March and October.

In recent years there has been a push to abolish DST transitions. The EU voted in 2019 to allow member states to stop switching, though implementation has stalled. The US Senate passed the Sunshine Protection Act in 2022 to make DST permanent, but it did not advance in the House. Several countries including Russia (2011), Turkey (2016), and Morocco (2018) have permanently adopted summer time, while others like Argentina and Brazil have abolished DST entirely.

Which Time Zones Observe DST?

Whether a time zone observes DST depends on national or regional legislation. Here is the current status of the 28 major time zones tracked by ZoneCross, computed at build time.

Zones like IST (India), JST (Japan), SGT (Singapore), and GST (Dubai) have never observed DST or abandoned it decades ago. The US state of Arizona (except the Navajo Nation) does not observe DST despite being in the Mountain Time zone.

How DST Affects Meeting Scheduling

For distributed teams, DST transitions are the single biggest source of scheduling confusion. The problem is not just that clocks change -- it is that they change on different dates in different regions. Here is why that matters.

Tips for Teams

• Schedule recurring meetings using a specific IANA time zone (e.g., "America/New_York") rather than a fixed UTC offset. Calendar apps will adjust automatically.
• Send a reminder to all participants one week before any DST transition, noting how the meeting time will shift for each person.
• Use the ZoneCross converter to check how a specific date maps across zones -- it accounts for DST automatically.
• For the 2-3 week US-EU gap period, consider temporarily adjusting the meeting time so no one is disproportionately affected.
• Keep a shared calendar with DST transition dates for every zone your team operates in. See our DST Calendar for a comprehensive reference.

Key Dates to Watch

Here are the next upcoming DST transitions for major time zones. These are computed at build time based on current IANA timezone data.

For a complete list of DST dates from 2024 through 2027 for every tracked time zone, see the DST Transition Calendar. To understand how US clock changes specifically affect calls with Europe and India, read How US DST Affects International Team Calls.