The International Date Line runs roughly along the 180th meridian in the Pacific Ocean. Crossing it westward moves you forward one calendar day; crossing eastward moves you back one.
It zig-zags to keep island groups and countries on a single date. That is why Samoa and American Samoa are 25 hours apart despite being neighbours.
What this means for scheduling
A Monday morning call in Los Angeles is already Tuesday morning in Auckland. Whenever a conversion here lands on a different calendar day, the result says so explicitly rather than leaving you to work it out.