Most of the US, Canada and Mexico's northern border cities will start Daylight Saving Time (DST) at 2 am (02:00) local time on March 14, 2021. The clocks will "spring forward" (by shifting the clock forward) an hour to daylight time, so that evenings have more daylight and mornings have less. The most of Europe will start DST on the last Sunday in March. Most of North America shifts at 02:00 local time, so its zones do not shift at the same time; for example, Mountain Time can be temporarily either zero or two hours ahead of Pacific Time.
Daylight saving time - also summer time in several countries - is the practice of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. The modern idea of daylight saving was first implemented during the First World War. Many countries have used it at various times since then.
The practice has been both praised and criticized. Adding daylight to evenings benefits retailing, sports, and other activities that exploit sunlight after working hours, but can cause problems for evening entertainment and other occupations tied to the sun. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how Daylight Saving Time currently affects energy use is limited or contradictory.
Starting in 2007, most of the United States and Canada observe DST from the second Sunday in March to the first Sunday in November, almost two-thirds of the year. The 2007 US change was part of the Energy Policy Act of 2005; previously, from 1987 through 2006, the start and end dates were the first Sunday in April and the last Sunday in October, and Congress retains the right to go back to the previous dates now that an energy-consumption study has been done.
DST clock shifts present other challenges. They complicate timekeeping, and can disrupt meetings, travel, billing, recordkeeping, medical devices, heavy equipment, and sleep patterns. Software can often adjust computer clocks automatically, but this can be limited and error-prone, particularly when Daylight Saving Time protocols are changed. (With material from: Wikipedia)