The International Date Line roughly follows which meridian?

The International Date Line roughly follows the 180th meridian.

The 180th meridian lies directly opposite the Prime Meridian and passes mostly through the Pacific Ocean. Because Earth rotates through 360 degrees in roughly 24 hours, longitude near 180 degrees is the natural region for separating calendar dates. Crossing the line generally changes the date by one day.

The International Date Line is not a perfectly straight line and is not fixed by a single treaty. It bends around national borders and island groups so that nearby communities and territories can share the same civil date. Large deviations occur around places including Russia, the Aleutian Islands, Kiribati, Fiji, and New Zealand.

A common mix-up is to treat the Date Line as identical to the 180th meridian. The meridian is a geographic line of longitude; the Date Line is a practical, irregular boundary for calendar conventions. The Prime Meridian at Greenwich, by contrast, is the reference point for measuring longitude and separates the Eastern and Western Hemispheres.