Cross International Date Line, At Sea

The International Date Line (IDL) is an imaginary line on the surface of the Earth, established by the International Meridian Conference of 1884, that runs from the north to the south pole and demarcates one calendar day from the next. It passes through the middle of the Pacific Ocean, roughly following the 180° longitude but it deviates to pass around some territories and island groups. 

 

The IDL is on the opposite side of the Earth to the Prime Meridian. The Prime Meridian is used to define Universal Time and is the meridian from which all other time zones are calculated. Time zones to the east of the Prime Meridian are in advance of UTC (up to UTC+14); time zones to the west are behind UTC (to UTC-12).The IDL and the moving point of midnight separate the two calendar days that are current somewhere on Earth. A traveller crossing the IDL eastbound subtracts one day, or 24 hours, so that the calendar date to the west of the line is repeated after the following midnight. Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day. The IDL is necessary to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.