Why GMT (Greenwich Mean Time) has been chosen as the basic time?
959716
Because Greenwich is where the Royal Observatory is. a more bookish explantion is belwo:\nIn 1675 King Charles II appointed John Flamsteed as the first Astronomer Royal to provide astronomical data for use by the Royal Navy. The Kings astronomer needed an observatory and so one was built at Greenwich. When away from land, Sailors find their position by taking the baring of a star, but the stars move across the sky during the night (actually the earth turns beneath them). To get their position they need to know the exact position of the star, at the time they took the baring. This information is given in books called navigational tables. So it is very important that sailors know what the time is, but the mechanical clocks they had did not keep time, because of the motion of the ship. In 1761 a man called John Harrison won a competition by making a clock which was accurate to 1/5th of a second a day, even at sea. From now on sailors could reliably find their exact position while out at sea.\n\nAs time went by, travelling long distances became less dangerous, so people traveled more and trade routes developed. New inventions like the steam train (1815) and the telegraph (1831) improved communications and the fact that different places were using different times started to become a real problem. So in 1884 twenty-five different nations held a conference in Washington DC, to discuss what to do. They split the world up into time zones and twenty-two nations voted to use the local time at Greenwich as the base time for the rest of the world.
740211
Japan is GMT +9 and Arizona is GMT -7. Add on 7 hours to get GMT and then add on the extra 9 hours and so you get 4 AM the following day.\n\nHope this helps,\n\nSupreme One
897781
GMT 0:00 is the first second of the day.\nAll countries times are eith + or - so many hours GMT.
82447
Greenwich, London, England is the home of Greenwich Mean Time (GMT), which is the Prime Meridian (zero longitude); here time is set for the world.
207628
We put the clocks forward by 1hr on Sunday 26th March 2006 and then put them back by an hour on the 29th October. It is basically to get more hours of daylight at decent times of the day!\n\nhttp://en.wikipedia.org/wiki/British_Summer_Time
9157
Time zones are areas of the Earth that have adopted the same standard time, usually referred to as the local time. Formerly, people used local solar time, originally apparent and then mean solar time. Their difference is the equation of time. Mean solar time is the average over the course of a year of apparent solar time (sundial time). With the expansion of the railways and as telecommunications improved this became increasingly awkward because clocks in a given town would differ from those in any other by an amount corresponding to their difference in geographical longitude, which was generally not a convenient number. Forcing all localities to run their clocks in synchrony would solve this problem, but under such a scheme, in many places the time shown by clocks at sunrise and sunset would differ too markedly from the solar time values to which people are accustomed. As a compromise, a scheme was devised where the surface of the planet was divided into twenty four "time zones", each separated by 15° of longitude and offset by one hour from its neighbor. Under this scheme, local time is always close to mean solar time, while comparing the time in different places is a simple matter of adding or subtracting whole hours. However, the one hour separation is not universal and, as the map below shows, the shapes of time zones can be quite irregular because they usually follow the boundaries of states, countries or other administrative areas.\n\nOriginally, time zones were aligned such that the Prime Meridian (longitude 0°) was the centre of its own time zone, with the mean solar time on that meridian (Greenwich Mean Time or GMT) defining its local time. As a result, all other time zones used GMT as the "base time", from which they were offset by a whole number of hours. However, as a mean solar time, GMT, or UT1 as it is also now known, is defined by the rotation of the Earth, which is not constant in rate. Initially, the rate of atomic clocks was annually changed or steered to closely match GMT, but effective 1 January 1972 the rate of atomic clocks became fixed, and time steps (leap seconds) replaced the rate changes. This new time system was called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth's rotation rate are confined to simple step changes that can be easily subtracted if a uniform time scale (TAI) is desired. With the implementation of UTC, nations began to use it in the definition of their time zones instead of GMT. As of 2005, most but not all nations have altered the definition of local time in this way (though many media outlets fail to make a distinction between GMT and UTC). Further change to the basis of time zones may occur if proposals to abandon leap seconds succeed.\n\nUTC is, incidentally, local time at Greenwich itself only between 01:00 UTC on the last Sunday in October and 01:00 UTC on the last Sunday in March. For the remainder of the year local time is UTC + 1, known in the UK as British Summer Time (BST). Similar circumstances apply in many places: see Daylight saving time.\n\nThe definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours.
1025233
Greenwich Mean Time does not change. The world is divided into 24 time zones going east (+) and west (-) from GMT. When we go from EST to EDT we are effectively changing our time zone from R (-5 hours) to Q (-4 hours).
453302
Its probably a GMT standard clock. Whereever you live is GMT + or - a certain amount of hours.\n\nEST is GMT -5 so if GMT says its 12:00, its actually 7:00 here.
771253
Actually time is measured in multiples of 24.  It may have something to do with our angular measurement system.  The Earth rotates 360° every 24 hours.  Each hour represents 15° of angular rotation.  Each minute of time represents 15 minutes of angular rotation and each second of time represents 15 seconds of angular rotation.  As to where our time measurement system came from, I don't know, but it could probably be found out pretty easily by doing a few searches.\n\nCheck this link for books on the subject:\nhttp://books.google.com/books?q=origin+of+time+measurement&ots=V7_ssegXJ4&sa=X&oi=print&ct=title
608394
All U.S. time zones are given relative to UTC (Coordinated Universal Time), which used to be called Greenwich Mean Time ... the time running through Greenwich England, used as the global standard reference.\n\nBecause of the U.S. Daylight Savings practices, the West coast of the U.S. sometimes uses PST and sometimes PDT.\n\nPST is UTC minus 8 hours, and is in effect during the winter (last Sunday in October to the first Sunday in April).\n\nPDT is UTC minus 7 hours and is in effect during the summer (first sunday in April to the last Sunday in October).
1136773
ANSWER from Eric Hammer on April 14, 1997: Since the Space Shuttle circles the Earth once every 90 minutes, the astronauts onboard have the unique \nopportunity of seeing many sunrise/sunset events every day. Therefore, unlike on Earth where we can look at the sun and get a rough idea of what time it is (before noon, after noon, dawn, dusk, etc.), the astronauts have no real reference of time. As a Flight Controller in the Mission Control Center, our primary measurement of time is Mission Elapsed Time (MET). This is a time system that begins from zero just as the Space Shuttle lifts off from the launch pad. All of our timelines and key events are referenced to MET, and thus that is the primary time reference for the astronauts as well. A secondary time reference is Greenwich Mean Time (GMT). This is a universal time system used around the world, and corresponds to the time in Greenwich, England. For example, it is 1010 local time in Houston right now. To convert that to GMT, I would have to add 5 hours, so it is 1510 GMT!
213340
PST is -8:00 GMT\n\nPST= Pacific Standard Time\nGMT= Greenwich Mean Time
1149914
Launch is planned for 15:49 local time, which is 19:49 GMT - note that the UK is *not* on GMT at the minute, but BST - a further hour ahead.
854107
Andyoptic is correct.  They use (now) Zulu time (Greenich, England time zone), when they need to relate to Earth time for something.\n\nHowever, most of the time, they are using the mission clock which simply starts at zero when they first lift off.\n\nHours-Minutes-Seconds only.  No days, weeks, months...
142218
<The 24-hour clock is a convention of time-keeping in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23 (and 24 in the day-ending midnight). This system is the most commonly used time notation in the world of today. The United States is the only industrialized country left in which a substantial fraction of the population is not yet accustomed to it. The 24-hour notation is in the US and Canada also referred to as military time, and (now only rarely) in the United Kingdom as continental time. It is also the international standard notation of time (ISO 8601).\n\n<Description\nA time in the 24-hour notation is written in the form hours:minutes (for example, 01:23), or hours:minutes:seconds (01:23:45). A leading zero is added for numbers under 10. This zero is optional for the hours, but very commonly used, especially in computer applications, where many specifications require it (for example, ISO 8601). Where subsecond resolution is required, the seconds can be a decimal fraction, that is the fractional part follows a decimal dot or comma, as in 01:23:45.678. In the 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day is that beginning at 23:59 and ending at 24:00. The time 24:00 of the current day is identical to 00:00 of the following day. Digital clocks run from 00:00 to 23:59, that is they never show 24:00 on their display. This way, the roll-over from 23:59:59.999 to 00:00:00.000 coincides with the start of a new day and date. However, the notation 24:00 is useful for referring to the exact end of a day in a time interval.\n\n24-hour clock 12-hour clock \n00:00 12:00 midnight \n01:00 1:00 am \n02:00 2:00 am \n03:00 3:00 am \n04:00 4:00 am \n05:00 5:00 am \n06:00 6:00 am \n07:00 7:00 am \n08:00 8:00 am \n09:00 9:00 am \n10:00 10:00 am \n11:00 11:00 am \n12:00 12:00 noon \n13:00 1:00 pm \n14:00 2:00 pm \n15:00 3:00 pm \n16:00 4:00 pm \n17:00 5:00 pm \n18:00 6:00 pm \n19:00 7:00 pm \n20:00 8:00 pm \n21:00 9:00 pm \n22:00 10:00 pm \n23:00 11:00 pm \n24:00 12:00 midnight
1115547
Primarily to provide consistency in what a certain time means everywhere.   Your day starts in the AM and ends in the PM.    We're all on the same 60 minutes, so ultimately it's not that difficult.
367588
time zone BST is a summer daylight savings time...it is one hour ahead of the universal time. only used in the summer time.. appears to give you 1 hour extra daylight time ...when the winter time comes around the time is switched back to greenwich mean time. hope this helps you out.