Why does the international date line deviate from 180th meridian for a portion of its length?
The International Date Line can be anywhere on the globe. But it is most convenient to be 180° away from the defining meridian that goes through Greenwich, England. It also is fortunate that this area is covered, mainly, by empty ocean. However, there have always been zigs and zags in it to allow for local circumstances. Over the years, the position of the International Date Line has changed several times. Until 1845, the Philippines were on the eastern side of it (the same side as the United States). It was on the eastern side of the line because it was a Spanish colony and most Europeans arrived there via the Spanish colonies in South America. Indonesia, almost directly to the South of the Philippines, was a Dutch colony and most European arrivals came via the Cape of Good Hope. Thus Indonesia was to the west of the International Date Line. After the independence of the South American countries, most people traveling to the Philippines also came by way of the Cape of Good Hope, so it
