So, in the case of the basement level referred to above, it may be given 0.000m as the site datum, in which case (following the logic from above) ground level = 3.250m ground floor = 4.000m first floor = 8.000m and so on.

For this reason, a suitable fixed point (called a temporary benchmark – TBM) is assumed to ensure that all other levels are positive. However, minus signs can be easily misread, not seen or read as a dash, so can cause confusion. In contrast, a basement which will be below ground will be below datum level and will therefore have a minus symbol prefixed to it e.g -3.250m. The datum on a site can be given a 0.000m designation but this does not have to necessarily be outside ground level if it is, a ground floor may be at say, 0.750m and a first floor at 4.750 and so on proceeding up the building. It can show the vertical height difference between floor levels of a building as well as differences in levels between one part of the site and another. This was decided at the International Meridian Conference in 1884 in Washington, D.C.On a construction project, a datum level is an arbitrary horizontal plane of reference from which all vertical dimensions are measured. The 180° meridian was selected as the basis for the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean. Unlike the International Date Line, whose course is determined by the countries bordering it, the Nautical Date Line is defined by international agreements. The International Date Line is de facto, which means that it is not based on international law and should not be confused with the Nautical Date Line, which follows the 180° meridian from the North to the South Pole. They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons. In 2011, Samoa changed its time zone from UTC-11 to UTC+13 by shifting the date line to the west and removing Decemfrom the calendar. By adding 24 hours to the the clocks in the Phoenix and Line Islands, they put a dent in the date line, moving it all the way to the 150° east longitude. In 1994, the Republic of Kiribati aligned the country's time zones by skipping Januin the portion of the country that was east of the date line. The International Date Line is not defined by international law, and it is up to the different countries to choose the date and time zone they want to observe. See it for yourself in our Time Zone Converter. Three Dates at the Same TimeĮvery day between 10:00 and 11:59 UTC, three different calendar dates are in use simultaneously on Earth.įor example, May 2 at 10:30 UTC, is 23:30 (11:30 pm) on May 1 in American Samoa (UTC−11), 06:30 (6:30 am) on May 2 in New York (UTC-4), and 00:30 (12:30 am) on May 3 in Kiritimati (UTC+14). For example, if you travel the 1061 km (659 mi) across the date line from Baker Island to Tokelau, you must add 25 hours, or 1 day and 1 hour.

See the date line on our Time Zone Map Not Always 24 Hoursĭepending on which time zone the country follows, the time difference on either side of the line is not always 24 hours. If you traveled from Kiribati's capital Tarawa to Hawaii's state capital Honolulu, you would have to turn the clock back 22 hours, effectively traveling back in time, at least on the calendar. The International Date Line is the boundary where each calendar day starts and is also known as the “Line of Demarcation” because it separates two calendar dates: When you cross the date line traveling east, you subtract a day, and if you cross the line traveling west, you add a day.įor example, Kiribati and Hawaii are on different sides of the date line. The Northern and Southern Hemispheres Different Days on Either Side For example, it leans towards the east at the Bering Strait between Asia and North America, leaving Cape Dezhnev in Russia a day ahead of Cape Prince of Wales in Alaska even though they are only 80 km (50 mi) apart. It is not straight but curves around landmasses and national borders. The date line runs from the North Pole to the South Pole and marks the Western and Eastern Hemisphere divide. It is halfway around the world from the prime meridian (0° longitude), the reference point of time zones, which runs through Greenwich, UK. The International Date Line (IDL) is located at about 180° east (or west). The International Date Line zigzags through the Pacific Ocean at around 180° longitude. Business Date to Date (exclude holidays).

Datum line definition