Does the international date line comprise a boundary between different days?

yes

Explanation:

The International Date Line (boundary) is from when each calendar day starts; west of the date line is 1 calendar day ahead (24 hour difference) of areas to the east.

The International Date Line runs from the North Pole to the South Pole along the Pacific Ocean; curves around the islands of Kiribati (to make the islands the same day), extends outward between Asia and North America, and causes Cape Dezhenev, Russia to be 1 day ahead of Cape Prince of Wales, Alaska and they are only 50 miles apart.

International Date line follows a 180 degree meridian (halfway around the globe from the prime meridian) and meausres 0-degrees longitude