Time zones and the date line

In short: The Earth rotates once in 24 hours. It is therefore divided into 24 time zones that differ roughly by one hour. At the date line the date changes.

In more detail: Universal time UTC is based on the prime meridian through Greenwich. Germany is in Central European Time (CET = UTC+1), in summer in daylight saving time (UTC+2).

In detail

• One hour per 15 degrees: 360 degrees divided by 24 hours.
• Deviations: Some countries use half-hour or 45-minute offsets, for example India (UTC+5:30).
• Date line: Roughly along the 180th meridian. Crossing it eastwards you gain a day, westwards you lose one.
• Clock changes: In the EU clocks are changed twice a year; abolition has been discussed for years.
• Programming: It is best to store times in UTC and convert only for display (jahreszeiten-entstehung).