Which longitude is taken as International Date Line and why?

Ans: The International Date Line (IDL) is an imaginary line on the surface of the Earth opposite the Prime Meridian where the date changes as one travels east or west across it. Roughly along 180° longitude, it mostly corresponds to the time zone boundary separating -12 and +12 hours Greenwich Mean Time (GMT). Crossing the IDL, travelling east results in a day or 24 hours being subtracted (so the traveller repeats the date) and crossing west results in a day being added. The exact number of hours depends on the time zones.