We all know what time is. It's the ticking of a clock, the whine of an alarm, the calendar on the wall. And since we all agree about how those things work, time can seem as solid as a rock.
Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.
Time of the day is related to the position of sun in the sky or its absence thereof. There is dawn, sunrise, early morning, morning, mid morning, noon, afternoon, late afternoon, evening, sunset, dusk, night and mid night. Then there are years, months, weeks, based on earth's yearly orbit around the sun and the changing seasons. The use of units like seconds and minutes which are radial angle measurements in geometry points toward the original connection of time measurements to radial motion of astronomical objects across the sky. Once we started using clocks, watches, and then digital time we got completely disconnected from the original method of measurement and time developed a life of its own.
Time can be defined from many perspectives. From perception view point time is an emergent concept which our mind creates. Present is the consciousness or awareness of recording of memory into the brain. Past is just a record while future does not exist. From point of view of physics time is presence of motion and forces in the universe. It involves all kinds of motion. The spin of particles and the motion of photons are time dependent. Gravitational force and electromagnetic forces are all part of time. As is the motion of celestial bodies the atoms and all other motion. We have partially understood the phenomenon of time. In the theory of relativity, the concept of time begins with the Big Bang the same way as parallels of latitude begin at the North Pole. You cannot go further north than the North Pole ,says Kari Enqvist, Professor of Cosmology.
We perceive times as flowing from the past through the present and into the future. We have memory of past events, but of course no memory of future events. Time provides us with a base line reference point in which events can be placed in order of occurrence, and in this manner we are able to establish that one event occurred before or after another, and this provides us with the so called 'arrow of time'. Interestingly, there is nothing in the laws of physics to suggest that time actually flows from the past through the present and into the future. So what is it that gives time a definite direction, the arrow of time? To seek the answer we need to examine the laws of thermodynamics. At a subatomic level there is no distinction between the past and the future. In a typical interaction involving subatomic particles, two particles may come together and interact in some way to produce two different particles, which then separate. According to the laws of physics there is no reason why these two new particles could not then interact and revert to their initial condition. By studying these particles it would be impossible to determine the order of events that took place, or indeed if any event had taken place. At this level there is no way to distinguish the past from the future simply by looking at each pair of particles.
Have a look and this amazing time-lapse video:
Time keeping is so critical to the functioning of modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as the International Atomic Time (TAI). Other scientific time standards include Terrestrial Time and Barycentric Dynamical Time. Coordinated Universal Time (UTC) is the basis for modern civil time. Since January 1, 1972, it has been defined to follow TAI with an exact offset of an integer number of seconds, changing only when a leap second is added to keep clock time synchronized with the rotation of the Earth. In TAI and UTC systems, the duration of a second is constant, as it is defined by the unchanging transition period of the caesium atom.
Greenwich Mean Time (GMT) is an older standard, adopted starting with British railroads in 1847. Using telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK. Universal Time (UT) is the modern term for the international telescope-based system, adopted to replace "Greenwich Mean Time" in 1928 by the International Astronomical Union. Observations at the Greenwich Observatory itself ceased in 1954, though the location is still used as the basis for the coordinate system. Because the rotational period of Earth is not perfectly constant, the duration of a second would vary if calibrated to a telescope-based standard like GMT or UT—in which a second was defined as a fraction of a day or year. The terms "GMT" and "Greenwich Mean Time" are sometimes used informally to refer to UT or UTC. The Global Positioning System also broadcasts a very precise time signal worldwide, along with instructions for converting GPS time to UTC.
Earth is split up into a number of time zones which are exactly one hour apart, and by convention compute their local time as an offset from UTC or GMT. In many locations these offsets vary twice yearly due to daylight saving time transitions. A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates (for example, the sun being at its highest point every day around noon), different places on the Earth need to have different clock times. Time zones have been used in modern times so similarly situated cities can keep exactly the same time, for simplicity and ease of communication.
Standard time zones could be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones would differ by one hour, and the variation in the position of the sun from one end of the zone to the other (east vs. west) would be at most 1/24th of the sky. Most of the 25 nautical time zones (specifically UTC-11 to UTC+11) are indeed defined this way, and are 15° of longitude wide. An hourly zone in the central Pacific Ocean is split into two 7.5° wide zones (UTC±12) by the 180th meridian, part of which coincides with the International Date Line.
On land, it is more convenient for areas in close commercial or other communication to keep the same time, so these zones tend to follow the boundaries of countries and their subdivisions instead. Of the 40 time zones on land, most are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC-12 to UTC+14), but a few are offset by 30 or 45 minutes from a nearby hourly zone.Daylight saving time is used in some higher-latitude countries to manipulate clock time with respect to the position of the sun for parts of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west relative to the corresponding nautical time zones, which also creates a permanent daylight saving time-like offset. Computer operating systems use either UTC or a local time zone to time stamp events.
Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Newfoundland, India, Iran, Afghanistan, Venezuela, Burma, the Marquesas, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as Nepal, and some provinces, such as the Chatham Islands, use quarter-hour deviations. Some countries, most notably China and India, use a single time zone, even though the extent of their territory far exceeds 15° of longitude. Before 1949, China used five time zones.
In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey.
Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age. Each era on the scale is separated from the next by a major event or change.
Greenwich Mean Time is still the legal time in the UK (in winter, and as adjusted by one hour for summer). But Coordinated Universal Time (UTC) (an atomic-based scale which is always kept within 0.9 second of UT1) is in common actual use in the UK, and the name GMT is often inaccurately used to refer to it. (See articles Greenwich Mean Time, Universal Time, Coordinated Universal Time and the sources they cite.)
Universal Time (UT) is the scale based on the mean solar day, defined to be as uniform as possible despite variations in Earth's rotation. UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources. UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. It varies from uniformity because of the irregularities in Earth's rotation.
Information sources: Wikipedia Time Physics Forbes