At 180 degrees longitude, the represents the point at which the eastern and western hemispheres experience different days.
a. equator
b. prime meridian
c. tropic of cancer
d. international date line

The area where the Eastern and Western Hemispheres meet but experience different days is known as the d. International Date Line 

It's actually not a straight line since it accommodates the land - so it goes around some places as not to split the date on land.

Answermaritime tropical;