Time Zones
Time zones tend to follow the boundaries between countries instead of strictly following longitude lines, because it is convenient for areas in frequent communication to keep the same time. Time zones are defined as offsets from Coordinated Universal Time (UTC).
Some time zones switch between Standard Time and Daylight Saving Time (DST).
Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time. It is not adjusted for Daylight Savings time. It is effectively a successor to Greenwich Mean Time (GMT).
Reference: Descriptions and links from Wikipedia.org
Pictured are major cities in each of the 24 hour time zones. Each zone is commonly referred to as GMT (Greenwich Mean Time) + or - 1 hour.
Longitude Lines
Longitude is a geographic coordinate that specifies the east-west position of a point of the surface of the Earth. Meridians are imaginary semicircular lines running from pole to pole. The prime meridian defines 0° longitude; by convention the International Reference Meridian for the Earth passes near the Royal Observatory in Greenwich, Great Britain. Positive longitudes are east of the prime meridian, and negative ones are west.
Because of the Earth’s rotation, there is a close connection between longitude and time measurement. Local time varies with longitude; a difference of 15° longitude corresponds to a one-hour difference in local time.
Reference: Wikipedia
World Clock “Longitude” Lines
Standard longitude lines pass through the center of each time zone. However, the longitude lines on the One World Clock are rotated 7.5° from the standard so that they define the boundaries of each time zone.
The One World Clock map includes a small triangle on the outer edge of the map indicating UTC 0° longitude (at the IERS Reference Meridian as the currently used Prime Meridian) and is not adjusted for daylight savings time. It is effectively a successor to Greenwich Mean Time (GMT).
Reference: Wikipedia