Each country has its own standardized time zone measured in hours east or west of Greenwich, spread in bands across the globe. Some countries have more than one time zone, e.g. the USA, Australia, Russia etc. The reason is that local mean time needs to be smoothed out over a fairly large geographical area to enable the coordination of everyone's clocks in that range. But real time is based on the rising and setting of the Sun, so it's not reasonable to expect that people in California should get up with New York people, when the Sun rises three hours earlier in the EST (Eastern Standard Time) zone than it does in California. In other words, when it is 7 AM in New York, it is 4 AM in San Francisco. Similarly, when it is midnight in Greenwich, it is 10 AM in Sydney, 8 AM in Perth and so on. In astrological calculations, however, we need to calculate the exact local time in order to find accurate positions of the planets and the rising sign etc so that the correct chart can be drawn up. Planetary positions in the ephemerides and houses in the tables are set according to UT (Universal Time, a more accurate version of GMT based on atomic clocks) which needs to be adjusted according to the local time, expressed in latitude and longitude. Ephemeris Time, an older way of reconciling the variability of the Earth's orbit, has not been used since Jan 1, 1984.