World Time - Astronomical Background

One day has exactly 23 hours, 56 minutes and 4 seconds, according to the rotation of the earth. Thereby the sun stands everytime anywhere in the world upright in the sky (the zenith) on the side of the world where is summer.
It isn't always exactly noon, when the sun stands in the zenith. This is because every country can choose its time zone itself. For example, huge China only has one time zone.

Daylight saving was introduced in different countries at quite various times. The intention was to have longer sunlight in summer in the evening, when people are more active. By this electricity should be saved, something that didn't work this good, but the daylight saving was kept up.

One year (the circulation of the earth around the sun) has 365 days, 5 hours, 48 minutes and 47 seconds. Because this is a bit unfunctional, a normal year was defined as 365 days and a leap year as 366 days. A leap year occurs every 4 years, not every 100 but again every 400. Therefore 2000 was a leap year and 1900 wasn't.