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International Date Line(IDL) is the line which is considered to be a start of a Day. In the above image a line is drawn in between American continent(right) and Asian Continent(left). This line is the point were the day is considered to start.
How does that affect?
Lets consider that the Sun is at top of the IDL. You might think both the continents on right and left has same date(lets consider the date to be 31 Aug(Today)). But in reality the continent on the right side has a date which is equal to Today-1(30 Aug). Still confused?. Ok, Lets say you start your flight from USA to Japan Today morning 10:00 AM(30 August) which takes 5 hrs to reach Japan. Now after landing you might think that the date must be still 30? No its 30. See below the time of two zones at the same time.
In order to correct this, whenever a flight crosses the IDL it either adds +1 or -1 to its date depending on the direction it travels. If the flight travels from Right to Left then it adds 1 day. If the flight travels from Left to Right then it subtracts 1 day.
Why is that so?.
Lets consider that the world to be a flat map as shown below. Everyone knows that Earth rotates from west to east and Sun rises in the east. So Eastern part gets the day first than the other parts. Hence as shown below, If the Sun is on the right side part of the world then the day has started there, while the previous day has not ended on the other parts of the world on the left. Thus making the difference.
Why the International Date Line is not straight?
This is to make all the territories of a country to have same time and date. That is if a country's border crosses the date line, then it cant have today(31 Aug) on left side and yesterday(30 Aug) on the right side.
If you still have any questions, then comment below.
Check this: Why Afghanistan is called as graveyard of Empires?.
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