The international date line is the 180th meridian of longitude, marking the point where one civil day ends and another begins.
It runs north and south across the middle of the Pacific Ocean.
In some places it veers a little to avoid lopping off parts of geographical locations such as Siberia.
Which way do you have to go to gain a day?
If you go west to east, for example, from Japan to California, you gain a day: Tuesday becomes Monday.
Go the other way and Monday becomes Tuesday.