Java Reference
In-Depth Information
In practice, time zones have been divided based on political regions: countries and regions within countries.
Local time in each time zone is an offset from UTC. The offset, the difference in UTC and local time in a time zone, is
called a zone offset. The regions east of the prime meridian use a positive zone offset. A negative zone offset is used
for regions west to the prime meridian. The zone offset is expressed in hours and minutes such as +5:30 hours,
-10:00 hours, etc. For example, India uses a zone offset of +5:30 hours; therefore, you can add 5 hours and 30 minutes
to UTC to get Indian local time. You may think that the value for a zone offset is fixed for a time zone. Alas, we, the
civilized and advanced humans, were so simple in timekeeping!
Some countries have more than one time zone. For example, USA has five time zones: Alaska, Pacific, Mountain,
Central, and Eastern; India has only one time zone. In USA, when the local time in Mobile, Alabama (Central time
zone) is 7:00 AM, it is 5:00 AM local time in Los Angeles, California (Pacific Time zone). Every part of India, because
of only one time zone, observes the same time.
The zone offset for some time zones vary in a year. For example, in Chicago, USA (called Central time zone),
the zone offset is -5:00 hours in summer and -6:00 hours in winter. Most countries use a fixed zone offset. For example,
India uses a +5:30 hours fixed zone offset. The rules for a time zone, governing the time when the zone offset changes,
and by how much it changes, are decided by government. These rules are known as time zone rules.
The zone offset ranges between +14:00 hours to -12:00 hours. If there are only 24 hours in a day, how do we have
+14 as a zone offset? The range, +14 to -12, makes it a 26 hours day! Note that some countries consist of several small
islands situated far apart on two sides of the International Date Line making them a day apart. This posed problems
in official communication among the islands of these countries as they observed only four common weekdays. They
extended the zone offset beyond 12:00, thus moving the International Date Line for their country, to keep the whole
country on one side of the International Date Line. Examples of countries using +13:00 and +14:00 hours of zone
offsets are Kiribati (pronounced as kirbas), Samoa, and Tokelau.
Daylight Saving Time is used to make better use of the daylight in the evening by moving the clocks forward
(usually by one hour) in spring. Clocks are set back by the same amount of time in fall. The period of the year during
which DST is observed is called summer; the other part of the year is called winter. Not all countries observe DST.
The government of a country decides whether the country (or only some locations within a country) observes DST;
if it does, government decides the dates and times to move the clocks forward and backward. For example, zones
observing DST in the United States of America moved the clocks one hour forward at 2:00 AM local time on March 11,
2012, thus creating a gap of one hour. Note that on March 11, 2012, the local time between 2:00 AM and 3:00 AM did
not exist in those zones in the USA. In fall, when clocks are moved back, a time overlap of the equal amount is created.
India and Thailand are two among several countries that do not observe DST. DST changes the zone offset of the DST
observing locations from UTC twice a year.
Calendar Systems
Humans use calendars to work with time. The time units used in calendars are year, month, day, hour, minute, and
second. In this sense, a calendar is a system of tracking time, past and future, for humans in a meaningful way for
social, political, legal, religious, etc. purposes.
Typically, a calendar system does not keep track of the time of day; it works in terms of day, month, and year.
Broadly speaking, in a calendar, a day is based on the rotation of earth on its axis, month on the revolution of the
moon around Earth, and year on the revolution of Earth around the sun. Sometimes, a calendar system is based on
week, which is based on a non-astronomical cycle.
Throughout the history of humankind, different civilizations have been known to use different calendar systems.
Most of the ancient calendar systems have been based on astronomical cycles generated by solar motion, lunar
motion, or both, thus giving rise to three types of calendrical systems: solar, lunar, and lunisolar calendars.
A solar calendar is designed to align with a tropical year (also called a solar year), which is the mean interval
between the vernal equinox. An equinox occurs twice a year when the center of the sun is in the same plane as
the equator of earth. The term “equinox” means equal night; on an equinox, day and night are almost of the same
length. A vernal equinox occurs during spring around March 21; an autumnal equinox occurs during autumn around
September 22. The Gregorian calendar, which is the most used calendar worldwide for civil purposes, is an example of
a solar calendar.
