Why is the library 'calendar' of great help in programming?

Note: There are 3 correct answers to this question.

Calculations of times are complex, as systems of 60 (minutes, seconds), 12 and 24 (hours), and 100 (milliseconds) are mixed.
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Due to the relativity of time, time is hard to measure.
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Dates are full of exceptions, like leap years or months of different length.
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Calculations with days are difficult because other units of times based on the number of days like week (7 days) or month (28 to 31 days) do not fit to each other.
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Due to different time zones, the calculation of time depends on where the computer is located.
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