Countries and Time Zones
Here you can see the different time zones and the according countries. The relativ time values show the calculation of the times in reference to the Western European Time (Greenwich Mean Time, GMT). Daylight saving time is not taken into account.
The time at a point on Earth depends on its angle to the sun. If it is perpendicular to the sun, i.e., at its shortest distance from the sun, then it is twelve noon. If it moves forward in time, it rotates towards the east, and the sun appears to move towards the west, and it becomes afternoon, evening, and finally night. Perpendicular again, but in the other direction, it is at midnight. After that, the sun appears in the east again, and it becomes morning. This is the true local time of a point on Earth. It is only the same for points that lie on the same longitude, i.e., have the same east-west coordinates. The north-south coordinates, i.e., the latitude, play no role in the local time. However, it would be very impractical if one had to use a different time for every movement east or west. In fact, on the equator, the time would then change by one second every 463 meters; the further from the equator, the smaller this distance becomes. The calculation for this is the Earth's rotational circumference / 24 / 60 / 60, i.e., divided by 86400.
This is why the world has been divided into time zones. It could have been divided into 24 time zones, each of which for one hour, with the line of true local time running through the middle of each. However, this would ignore both natural boundaries and national borders. Therefore, the boundaries of time zones sometimes follow straight lines from pole to pole and sometimes national or cultural boundaries or coastlines. Every country has the right to choose its own time zone(s). Large countries, at least those with a wide east-west span, often have multiple time zones within their territory. Exceptions here are China and India. Daylight saving time, which exists in many countries, moves a country or region into an inherently incorrect time zone, as if it were one hour, or 15 degrees of longitude, further west.