All territory of our planet is conditionally divided into 24 time zones, in each of which at some point day - own time. How to understand what time difference between two geographical points?
The time zone represents the certain territory extended along a meridian on which all square identical time acts. The necessity of formation of time zones was caused by attempt, on the one hand, to consider the movement of the planet Earth, and on the other hand - to take into account geographical remoteness of certain settlements from each other which causes unequal change of times of day in them.
Reference point
In turn, for ensuring comparability of time zones in the territory of all planet it was necessary to choose a certain reference point in relation to which time in all other settlements and regions would be defined. Originally as such point there was a so-called Greenwich meridian passing through London where there was a royal observatory. Time on this meridian was accepted for the zero time zone with the designation GMT. However in 1972 instead of time across Greenwich which unevenly considered the movement of Earth the World coordinate time which was more exact was entered. Since then this time designated by UTC is a reference point for time zones around the world.
Definition of time
Today the zero time zone where time to equally World coordinate time, acts on the territory of Greenland, Iceland, some islands belonging to Great Britain, the Norwegian island of Buva and a number of the African states. Shift of time zones of rather World coordinate time is positive and negative. So, at the movement from the zero time zone on the West shift will be positive, on the East - negative.
Calculation of time difference
Thus, to define what time difference between two settlements or other geographical points, it is necessary to define for a start to what time zone each of them belongs. After that it will be rather simple to perform necessary calculation. For example, we need to calculate time difference between Novosibirsk and Rio de Janeiro. For this purpose it is enough know that Novosibirsk is in the time zone of UTC+7, and Rio de Janeiro - UTC-3. From this it follows that the time difference between these cities is 10 hours. For example, when in Novosibirsk - 10 o'clock in the morning, in the capital of Brazil there comes midnight. The same way it is possible to define time difference for all other cases.