Time zone lines are not always straight in order to accommodate the desires of nations within the boundaries of the zone. The irregularity is mainly due to political factors and has nothing to do with geography or any other natural cause.
Time zone lines, when not deflected for political reasons, are 15 degrees of latitude in width. The boundaries are arranged over both land and sea. Each line is one hour earlier than the last line to the east, but within the boundaries of the time zone itself, time is not set earlier in the west than in the east.
Many countries in South America deflect time zone lines around their boundaries so that the entire nation is within the same time zone. Some nations not only deflect the lines around their borders but ignore any lines passing within their borders entirely. For example, China, which is about as wide as the continental United States, only has one time zone. Countries to the north and south of China follow the regular 15-degree-wide time zone lines, with some deflection.
Originally, China recognized five time zones, but in 1949, Chairman Mao Zedong issued a decree stipulating all of China to be on Beijing time in order to promote a sense of national unity. India instituted a similar policy upon achieving independence. Although Beijing time is the only officially recognized time within China, certain ethnic groups residing far west of Beijing utilize regular 15-degree-wide time zones to assert ethnic pride.