timezones

I've always been confused about these.

Imagine picking a line of longitude \(l\). Draw a red line along that longitude. Let's start the new day there. When the 21st starts there, everywhere else on earth, it is the 20th. It has been the 20th for 24hrs at \(l\). 15 degrees to the west, it has been the 20th for one hour. 15 degrees to the east, it has been the 20th for 23hrs. On the other side of the world, it has been the 20th for 12hrs. The other side of the world is behind by 12hrs.

How do you know if you are ahead or behind of another location? Whoever gets hit first by the sweeping dateline for the 21st. So starting at \(l\) and moving westward, who gets hit first? If there is less than 180 degrees (sweeping westward) between the two locations, the easternmost location is ahead.