What imaginary line, near 180 degrees longitude, marks the conventional boundary between calendar dates?

The International Date Line is the imaginary boundary near 180 degrees longitude where the calendar date conventionally changes. Crossing it generally moves the date forward or backward by one day.

The line is not perfectly straight because it bends around national territories and island groups. These adjustments help countries and communities keep the same calendar date across their territory or with nearby neighbours. The line also passes through the Pacific Ocean, where there are relatively few large land areas.

The International Date Line is a civil-time convention rather than a physical feature or an internationally fixed legal border. The Prime Meridian at Greenwich is the reference for longitude, while the Equator divides Earth into northern and southern hemispheres. The two sides of the Date Line can therefore have different dates even when locations are geographically close.