Daylight Saving Time (DST) is the practice of moving clocks forward by one hour in spring and back by one hour in autumn. The goal is to make better use of natural daylight during the longer days of summer. The mnemonic "spring forward, fall back" captures the direction of the change.
A brief history of DST
The idea of adjusting clocks to save daylight was first proposed by New Zealand entomologist George Hudson in 1895. He wanted more after-work hours of daylight for his insect-collecting hobby. The concept was independently promoted by British builder William Willett in 1907. Germany became the first country to adopt DST in 1916 during World War I to conserve coal. The United States followed in 1918. Over the decades, DST adoption has expanded and contracted as countries weigh energy savings against the disruption of changing clocks.
Which countries observe DST?
About 70 countries observe some form of DST, covering roughly 1.6 billion people. The main adopters are in North America and Europe. The United States (except Arizona and Hawaii), Canada (except Saskatchewan), the European Union, the United Kingdom, Australia (New South Wales, Victoria, South Australia, Tasmania, ACT), and New Zealand all observe DST. Most of Asia, Africa, and South America do not observe DST. Notable recent changes include the European Union debating abolition since 2019, and several US states passing legislation to make DST permanent (pending federal approval).
When do clocks change?
Transition dates vary by region. In the United States and Canada, clocks spring forward on the second Sunday of March and fall back on the first Sunday of November. In the European Union and UK, the shift happens on the last Sunday of March and last Sunday of October. In the Southern Hemisphere, the seasons are reversed: Australia springs forward on the first Sunday of October and falls back on the first Sunday of April. New Zealand transitions on the last Sunday of September and the first Sunday of April.
How DST affects timezone conversions
DST creates temporary shifts in the offset between time zones. For example, the normal difference between New York (EST, UTC-5) and London (GMT, UTC+0) is 5 hours. But when the US springs forward to EDT (UTC-4) before the UK moves to BST (UTC+1), the gap temporarily narrows to 4 hours. A few weeks later, when both have transitioned, the gap returns to 5 hours. During the autumn transition, a similar mismatch occurs. These transition periods are the most error-prone time for international scheduling.
The case for and against DST
Supporters argue that DST reduces energy consumption (less need for artificial lighting in evenings), promotes outdoor activity and retail spending, and reduces traffic accidents and crime during evening hours. Critics point out that modern energy usage patterns (air conditioning, electronics) may negate savings, that the time change disrupts sleep and increases heart attack risk in the days following the spring transition, and that the biannual disruption causes confusion in international business. Studies on energy savings are mixed, with some showing modest reductions and others showing no effect.
Tips for handling DST transitions
Always verify timezone conversions in the weeks around DST transitions. Use tools that account for DST automatically (like our timezone converter). When scheduling recurring international meetings, be aware that the time difference may change by an hour during transition periods. Set reminders to check your meeting times when DST changes are approaching. For software developers, always store times in UTC and let libraries handle the local conversion — never manually add or subtract DST offsets.