At some time between 1:00 PM and 3:00 PM, it will be 2:00 PM exactly once.

[cpuwzd]

I noticed this problem while driving from my brother's home in the Central Time Zone to my home in the Eastern Time Zone. I inevitably cross between time zones between 1:00 PM and 2:00 PM Central Time and my medication reminder never goes off at 2:00 PM because the hour skipped includes 2:00 PM.

There's also uncertainty about where the line falls when you are using GPS to determine your location. I don't ever remember seeing a road sign that tells me that I have just changed time zones. You can't depend on state line markers because in places like Indiana, the time zone boundaries are mostly political or economic.

What point of your body determines which time zone you are in? When you are moving, this affects the timing of your time zone change. Looked at another way, which point of your cell phone determines which time zone it is in?