Question

A person flying to San Francisco from Kathmandu crossing the IDL gains a day, but his/
her friend flying to Kathmandu from the same place crossing the line loses one day. Why?​

Answer 1

Answer:

Think about the way time zones work. If it is afternoon or evening and you start looking at the time of day in the time zones East of you, eventually you will find a time zone where it is between 11 pm and midnight today, and then you will see that in the next time zone over, the time is between midnight and 1 am tomorrow. The date changed from today to tomorrow when you crossed the midnight line.

As you keep travelling East around the world from the “1 am tomorrow” time zone back to where you started, the day has to change back to “today” somewhere. The governments of the world have all decided to agree that this is always the same place, the International Date Line (IDL).

The date isn’t a natural thing. Its an artificial concept that needs an IDL or it won’t work.

The time in Katmandu is generally 12 hours and 45 minutes later than in San Francisco. If the time in Katmandu is between 12:45 in the afternoon and midnight, then Katmandu and San Francisco are on the same date. When you look from one to the other, your gaze crosses both the IDL and the midnight line, so your gaze both “gains” and “looses” a day.

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