Both of the guys are born on the same day, month, and year but they have different ages.
One of them is 5 months, 12 days, and 11 years old and the other is 2 months, 17 days, and 11 years old.

Why are there two different ages then?

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> They were born at the same time.
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> There is no third child.
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> They were born on the same time and place.
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HINT 1:

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>  They might not be on the same page here...
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HINT 2:

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>  Now that people have guessed the idea of the answer, it is one of the most popular calendars in the world. (See comments for info)
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