What is date line?

When the last ship of Magellan's expedition reached home after sailing around the world, there was an interesting discovery: the last entry made in the ship's log deviated from that of the people left behind by one day. After much speculation, scientists came to the conclusion that every time a person journeys around the Earth, the date upon return would be different form the one calculated according to standard time. They realised that it was necessary to fix a certain date line for such expeditions according to which the correct log date could be calculated. This 'line' was drawn through a stretch in the Pacific Ocean where it was not divided by any continuous landmass. It is an imaginary line, which goes zigzag from the North to the South Pole along the 180' longitude. The seafarer's watch had to be set backwards by 24 hours once the date line from west crossed, so that the same date would apply to 2 days.