Season Countdown

DateMath counts to the moment of the equinox or solstice, from the US Naval Observatory's table in UTC to the minute. That moment is the same everywhere, so the countdown reaches zero at once in every time zone, and DateMath shows it on your device's clock. Astronomical seasons start at these moments; meteorological seasons start on the 1st of March, June, September and December. The seasons are the Northern Hemisphere's; south of the equator they are reversed.

• Equinoxes and solstices. DateMath takes the moment of each equinox and solstice from the US Naval Observatory's table, in UTC to the minute. The holidays by date tool dates it on the US Eastern clock, and the season countdown shows it on your own clock. Near midnight the date differs between time zones. Source: USNO Earth's Seasons.
• Your device's local time. DateMath's countdowns read the current time from your device and count to the target in your own time zone. In the US, the standard time zones are set by 15 U.S.C. 261 and daylight saving time by 15 U.S.C. 260a (from 2:00 AM on the second Sunday in March to 2:00 AM on the first Sunday in November); your device applies them. Source: 15 U.S.C. 261, 15 U.S.C. 260a.
• US federal holidays. DateMath skips the 11 annual holidays of 5 U.S.C. 6103 on the days they are observed for federal employees, as the US Office of Personnel Management (OPM) lists them: a Saturday holiday on the Friday before, a Sunday holiday on the Monday after. Source: 5 U.S.C. 6103, OPM federal holidays.

DateMath's date math is tested against more than 100 cases worked out by hand. Every worked example and table on the site is computed with the same code the calculators run, so a page and its calculator can't disagree.

A season starts at an equinox or a solstice, a moment the US Naval Observatory gives to the minute. The countdown above runs to the next one on your own clock; the table gives the coming four on the US Eastern and Pacific clocks and in UTC:

Each season has a countdown of its own: spring countdown, summer countdown, fall countdown and winter countdown.

There are two ways to date the seasons, and both are in use. Astronomers start each season at an equinox (day and night about equal) or a solstice (the Sun at its farthest north or south), so the start moves by a day or so from year to year. Weather services such as NOAA use whole months instead, so that the records of one season compare with the next.

The countdown counts to the astronomical start, the one calendars print, and shows the meteorological start under it. In the US, summer also has an unofficial start, Memorial Day weekend, which the summer countdown adds.

The seasons are not the same length, because Earth's orbit is an ellipse: it moves slowest when it is farthest from the Sun, in early July, so the Northern Hemisphere's summer is the longest season and its winter the shortest. The lengths from the USNO moments:

An equinox or a solstice is one moment for the whole Earth, so this countdown reaches zero at the same instant everywhere; only the clock time it shows changes with your time zone. Near midnight the date changes too: an equinox late in the evening on the Pacific clock can already be the next day in UTC. The holidays by date tool lists each season on its US Eastern date, and the time zone converter turns any of the times above into another zone.

South of the equator the seasons are reversed: the June solstice starts winter in Australia, Argentina and South Africa, and the December solstice summer. The countdown uses the Northern Hemisphere's names, as the US does. To count down to another day of the year, such as the last day of school, use the day countdown.