Ah yes, the IDL. From our trusted Wikipedia:
The International Date Line (IDL) is an imaginary line on the surface of the Earth opposite the Prime Meridian which offsets the date as one travels east or west across it. Roughly along 180° longitude, with diversions to pass around some territories and island groups, it mostly corresponds to the time zone boundary separating +12 and -12 hours GMT (UT1). Crossing the IDL traveling east results in a day or 24 hours being subtracted, and crossing west results in a day being added.
Knowledge is power!
Sylvia