Time is one of the external forms of substance existing. The course of change 、 movement and development of any matter always takes place within time and space

Time; it has two meanings: one is “ a period of time ” ,it represents the interval between two instantaneous points; the other is “ the time of day ” , it represents a instantaneous point. For example, we go to work at eight o ' clock in the morning, rang out at twelve o ' clock, we work four hours in the morning. The “ four hours ” is “ a period of time ” , and the eight o ' clock and twelve o ' clock are “ the time of day ” .

Time is measured by accumulation of the same periods. As people discovered earlier, the movement of the sun that has a close relation with human life is very regular, and that this kind of movement is the result of rotation and revolution of the earth, therefore, a timing system which measures time by the movement of the sun is established, it takes the periodical time of sunrise and sunset as a standard to measure time.

Solar day is defined according to the periodical time of rotation and revolution of the earth. Solar day includes “ true solar day ” and “ mean solar day ” . True solar day is a “ day ” that is measured by the sun, namely the time interval between the sun appearing in the middle sky and it next appearing there, is called a true solar day. But the periods between two true solar day are not always equal, in order to measure time more regularly, “ mean solar day ” is obtained by averaging total time of the movement of the earth by astronomers. We live “ mean solar day ” in daily life. In the mean solar day system, the midnight moment is taken as zero moment of mean solar day, also called mean midnight. The time that the earth completes one revolution is one year, the time that the earth completes one rotation is a day, a day is divided into twenty-four hours, an hour is divided into sixty minutes, and a minute is divided into sixty seconds, “ second ” is the basic unit of time, if a mean solar day is averagely divided into eighty six thousand and four hundred periods of time, the length of each period is the basic unit of mean solar day system ─ the length of mean solar second: One second= 1/86400 mean solar day.

Normal time and standard time. Each observation point on the ground has its own mean midnight moment, in the same moment, observation points on different longitude get different time, it is called local time. The rotation of the earth takes twenty-four hours, the east and west longitude totals up to three hundred and sixty degrees. So the time difference is one hour every other fifteen degrees （ 360 ° +24=15 °） , if each place uses its own local time, there would be much inconvenience, therefore, standard time is regulated. The zero longitude that goes through the Greenwich astronomical observatory in London is set as the center longitude in the zero zone, the zone between west longitude 7 ° 30 ' and east longitude 7 ° 30 ' is defined as zero zone. In this time zone, the standard time is the local time of the zero longitude . This is Greenwich Time, and is also called universal time. From the border line of zero zone to the east and the west, every fifteen degrees of difference of longitude is a time zone, twenty four time zones all together, with one hour time difference each time zone. The standard time of each zone is the local time of the center longitude of this zone, for example, the position of our capital Beijing is longitude 116 degrees east, the eighth zone east, the center longitude of this zone is east longitude 120 ° , so “ Beijing Time ” is actually the local time of east longitude 120 ° .

In fact, the borderline of time zone is usually set according to the district or natural boundary of each country for the sake of convenience. Our country has extensive territory, stretches across four time zones, but only one zonetime is adopted at present, namely “ Beijing Time ” is the uniform standard time of our country. Since zero time and the twenty-fourth hour are superposed, in order to differentiate today and tomorrow, a line near longitude 180 ° is set as international date alteration line, when people navigate across the line from different directions, they need to conform to the rule to shift to an earlier or an later date.