Daylight saving time (DST) begins Sunday morning at 2 a.m. standard time when clocks are moved forward to 3 a.m. Hawaii and Arizona (except for a portion of the Navajo Nation in Arizona) do not follow DST. The practical effect of DST is to prolong evening daylight at the expense of morning daylight. Of course, the amount of daylight is not changed. The daylight hours are just shifted with respect to our civil timekeeping.
Our modern calendar is based on the sun. It defines a day as consisting of 24 hours and a year as consisting of 365 days. Unfortunately, the orbital period of the Earth is actually 365.24219 days, producing a gradual drift of the seasons with respect to a 365-day calendar year.
Our present calendar was introduced by Pope Gregory XIII in 1582 and finally adopted by Britain and the American colonies in 1752 to supersede the Julian calendar introduced in the Roman Empire in 46 BC by Julius Caesar. The Julian calendar produces an error of eight days in a 1,000 years.
In it, all century years (1600, 1700, et cetera) were leap years. In the Gregorian calendar, only those century years divisible by 400 are leap years. For example, 2000 was a leap year, but 1900 and 2100 are not leap years. The Gregorian calendar is accurate to about one day in 3,000 years. No need for a calendar change for a while.