There are four kinds of time, these are
1. Sidereal time
The measurement of sidereal time is based upon the divinal motions of a star or the vernal equinox. The time interval between two successive upper transits of the vernal equinox over the same meridian is called Sidereal Day. The Sidereal Day is divided into 24 hours, each hour is divided into 60 minutes and each minute is divided into 60 seconds. The right ascension of the meridian of a place is known as local sidereal time.
2. Apparent solar time
Apparent solar time is the time derived from the Sun’s apparent position. Its measurement based upon the daily motion of the Sun. The time duration between two successive lower transits of the center of the Sun over the same meridian is called an Apparent Solar Time.
3. Mean solar day
When time measured by the divinal motion of the near Sun is called the Mean Solar Time. The mean Sun is an imaginary point and is assumed to move at a uniform rate along the equator so as to make a solar a day of a uniform length. The motion of the means Sun being the average of that of the true Sun is right ascension.
4. Standard Time
In order to avoid confusion arising from the use of different local mean time by the people, it is necessary to adopt the mean time of a particular meridian as the standard time for the whole country. This meridian is known as the standard meridian and usually lies an exact number of hours from Greenwich. The meantime associated with this meridian is called the standard time, which is kept by all watches and clocks throughout the country.
For a country like Russia which extenteds from 90o east latitude to 169o West latitude, It is practically impossible to adopt a single Standard Time. Hence, it follows 11 time zones, USA and Canada have 5 time zones each.
Local time can be reckoned by the shadow cast by the Sun, Which is the shortest at noon and largest at sunrise and sunset. Towards East of Greenwich line, time increases at the rate of 1o = 4’ or 15o = 60’. Similarly, to the west, time decreases with same rate. If it is 12 noon at Greenwich, the time at 15o east would be 1 p.m. and 15o West would be 11 a.m. Taking Green it material as a standard time zone (GMT-Greenwich Mean Time), the whole world has been divided into 24 standard time zones, each of which differs from the next by 15o in longitude for one hour in time, since the Earth rotate by 360o in 24 hours.
The Earth takes 4 minute to move by 1o of longitude and 60 minute to move by 15o of longitude. Since, the sun rises in the East and Earth moves from West to East, time on Eastern hemisphere is ahead of the Greenwich and that of the Western hemisphere is behind it.
Indian Standard Time
For a country like India which extent from around 68o E to 97o E, it is necessary to adopt the local time of some central meridian of a country as the standard time for the whole country.
In India the longitude of 82o 30 minute East is treated as the standard Meridian. The local time at this Meridian is taken as a standard time for the whole country. it is known as Indian Standard Time. it is passed through Mirzapur near Allahabad(UP).
Daylight savings time This is basically practiced in temperate regions. Also summer time in several countries is the practice of advancing clock so that evening have more daylight and morning have less. Typically clocks are adjusted forward 1 hour near the start of spring and are adjusted backward in autumn. Adding daylight to evening benefit retailing, Sports and other activities that exploit sunlight after working hours, but can cause problems for evening entertainment and other occupation 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 uses patterns differ greatly and reserved about how DST currently affect energy use is limited or contradictory
International Date Line
International Date Line marks the place where each day officially begins. At the international date line, the west side of the line is always one day ahead of the East side, No matter what time of day it is when the line is crossed. The International Date Line has no astronomical or physical meaning. it is an imaginary line drawn at the 180o longitude avoiding the continuous line parts.
it is bent at 75o north Latitude toward East to avoid division of Siberia and to separate Siberia and Alaska. Again, it is bent over Bering Strait toward West .To maintain continuity of land in the Island of Fiji and New Zealand, the International Date Line bends towards east in the southern Pacific Ocean. There is a difference of 24 hours or one day in East and West of it therefore, 1 day either gained or lost while crossing this line.