A lot of the current technology to work properly requires a very precise definition of the time. Typically this time is maintained by specific atomic clocks and has a satellite distribution.
There are many standard definitions of the time, but the main four are: UTC, GPS, LORAN and TAI.
UTC time - Coordinate Universal Time
Coordinated Universal Time is the time zone "0" from which all other time zones around the world are then calculated. It is similar to Greenwich Mean Time (GMT), but unlike this time, it is not based on the time of rotation of the Earth, but on measurements made by atomic clocks that are most accurate and above all more stable over time.
Simplifying, the UTC time divides time into days, hours, minutes and seconds, but while one day is always of 24 hours and one hour always of 60 minutes, a minute, while almost always consists of 60 seconds, sometimes it can be a period of 59 or 61 seconds: this variation depends on the time of rotation of the earth around the sun, which, due to fluctuations in its velocity, is not constant. With this change in duration of minutes, you do so in order to keep the delay between the UTC and the "earth time" within 0.9 seconds: when needed (typically on June 30th or December 31st), on the recommendation of the International Earth Rotation and Reference Systems Service (IERS, which deals, among other things, the measure of Earth's rotation), is considered a minute with duration of 59 or 61 seconds. It is typically taken off one second every 18 months, although of course this is not a certain and scientific rule. This second is called "leap second" and its calculation starts from 1972.
Sometimes the UTC time is also called "Zulu time", simply for the fact that time is briefly referred to as "Z time" in naval-air navigation and the letter Z is articulated as "Zulu" in the NATO phonetic alphabet.
The GPS time is based on atomic clocks and has no correction with respect to proper time variations of the Earth's rotation: the only corrections made on atomic clocks located on satellites are very small adjustments to ensure that they remain perfectly synchronized with atomic clocks installed on the Earth (usually to correct drifts due to relativistic effects). The time "0" which first GPS timing started is midnight on January 6th, 1980 (UTC time): in that year, the GPS time coincided exactly with UTC time, as a result of corrective measures to UTC time, today it differs by 16 seconds compared to GPS time, which as written above, is not subject to any astronomical adjustment.
The date received from GPS is based on the instant "0" just mentioned: in substance, the system transmits the number of weeks spent by January 6th, 1980 and the number of seconds since the beginning of the current week. The weeks are transmitted with a 10-bit encryption, which means that the number of weeks spent are reset every 1024 weeks (about 19.6 years) and the last reset (ant for now the only one) has been on 21st August 1999.
At this moment, therefore, the GPS system transmits the following values:
The LORAN is a terrestrial radionavigation system essentially used to determine the location of ships and aircrafts on the globe, the acronym stands for Long Range Navigator.
Although it is designed to handle positioning systems, this system (like GPS) is able to provide very precisely "time Loran", which is a time-based on atomic clocks (especially those installed on the transmitter chain of LORAN-C) whose time zero starts January 1st, 1958. Even the time LORAN (as well as GPS time) is not affected by any astronomical time correction (as, on the contrary,the UTC is), and therefore differs from UTC by several seconds (this difference is currently 25 seconds), corresponding to "leap seconds" introduced in 1972.
At this moment, therefore, the LORAN system sends the following time:
TAI time - International Atomic Time
International Atomic Time is based on the average time maintained from over 200 atomic clocks located in about 70 national laboratories in various parts of the world, again without introducing any astronomical correction. Although the first of these atomic clocks began to be operational in 1955, the zero was set at January 1st, 1958; differently from the LORAN, however, it is assumed that from 1958 to 1972 (date of introduction of the "leap seconds") there should have been 10 other "leap second" to be introduced: this is why this time differs from UTC time by 35 seconds instead of the 25 seconds from the time that differs from LORAN.
At this moment, therefore, time TAI is: