The biggest time difference on land is between China and Afghanistan, 3 and a half hours, but there is no border crossing point there: https://en.wikipedia.org/wiki/Wakhjir_Pass
AFAIK The biggest time difference where you can cross is between Tajikistan and China, you would have to set your clock by 3 hours. There is one border crossing there Kulma Pass
Kiribati switched to the other side of the date line in 1994, so there are 3 different days at the same time on the Globe for an hour:
The biggest time difference on land is between China and Afghanistan, 3 and a half hours, but there is no border crossing point there: https://en.wikipedia.org/wiki/Wakhjir_Pass
AFAIK The biggest time difference where you can cross is between Tajikistan and China, you would have to set your clock by 3 hours. There is one border crossing there Kulma Pass
Kiribati switched to the other side of the date line in 1994, so there are 3 different days at the same time on the Globe for an hour:
https://www.timeanddate.com/worldclock/converter.html?iso=20231014T103000&p1=274&p2=136&p3=1890
So be careful when you ask what day it is