Which border has the greatest time difference as you cross it? The greatest difference I've found is Afghanistan/China, with a difference of 3½ hours. Afghanistan's time zone is GMT+4½ and China's is GMT+8. You could almost get jet-lag walking over the border.
Based on longitude, in the west of the Afghanistan, GMT+4 would be appropriate, and in the extreme east at the Chinese border, GMT+5 would be appropriate. Therefore, Afghanistan's national time zone of GMT+4½ is perfect. However, based on longitude, China's time zone in the extreme west at the Afghan border ought to be GMT+5, and in the far east it ought to be GMT+9. China's single time zone of GMT+8 for the entire country causes it to remain light until very late in the evening in the west of the country. It's just like California always being on New York time.