The international date line is defined as an imaginary line that goes north and south through the Pacific Ocean, one day is on the east side of the line and the following day is on the west side.
An example of the international date line is when it's 3pm on Sunday in Samoa, it's 3pm on Monday right across the line, just 550 miles away in Tonga.
An imaginary line running north and south through the Pacific Ocean, largely along the 180th meridian: there is a 24-hour time difference between a point just west and one just east of the line, so that when it is Sunday just west of the line, it is Saturday just east of it.
An imaginary line through the Pacific Ocean roughly corresponding to 180° longitude, to the east of which, by international agreement, the calendar date is one day earlier than to the west.
An imaginary line on the Earth's surface that is internationally agreed upon as the place where each new calendar day begins. The line extends from the North to the South Pole through the Pacific Ocean, roughly along the 180th meridian. The calendar day to the east of the line is one day earlier than it is to the west of the line. The International Date Line was established at the International Meridian Conference in 1884 in order to standardize time, especially for the purpose of travel.