The International Date Line is an imaginary line running roughly along the 180th meridian, opposite the Prime Meridian in Greenwich. Crossing it means adjusting the calendar date by a full day, in addition to any time zone adjustment.
Why it exists
Because the world is divided into time zones roughly 15 degrees of longitude wide, going all the way around the globe means you'd eventually gain or lose a full 24 hours if there weren't a fixed point where the date resets. The date line serves as that reset point, keeping the global calendar consistent.
Why it isn't a straight line
Rather than following the 180th meridian exactly, the date line zigzags around several island nations and territories specifically so that a single country doesn't get split across two different calendar dates. Kiribati, for example, adjusted its portion of the line in 1995 so the entire country could share the same date, briefly making some of its islands among the very first places on Earth to reach each new day.
What happens when you cross it
Traveling westward across the date line (for example, flying from Los Angeles to Auckland) skips a calendar day forward. Traveling eastward across it (Auckland to Los Angeles) repeats a calendar day. This is separate from, and in addition to, any change caused by crossing standard time zone boundaries during the same flight.
A famous historical example
Samoa switched to the western side of the date line in December 2011, meaning the country skipped December 30 entirely that year, jumping straight from December 29 to December 31. The change was made to align Samoa's business week more closely with its major trading partners, Australia and New Zealand, rather than the United States.
Practical takeaway for travelers
If your flight itinerary shows you landing before you took off, or a full day later than expected, check whether your route crosses the International Date Line — it's almost always the explanation, not an error in the ticket.