Remove unused imports

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Calendar constants are calculated values.

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Add  more conversion functions.

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Correct conversion of microseconds to seconds.

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Add missing test for converting times.

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Convert all parts of a datetime object.

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Delegate calculations to functions not class methods.

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Remove own conversion code.

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Handle dates.

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Convert fractional parts of days to hours, minutes, seconds and microseconds.

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Avoid rounding erros by working directly with the fractions of  the day.

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Fix conversion of Excel values < 1 to time objects.

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Test adjusted to reflect how Excel handles days.

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Add test to show that Excel mucks around with dates < 1900-02-28

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Adjust for dates less than 1900-02-28 and reinstate test values.

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Removed test in test_cell.py that claimed to be working with strings when it actually is working with dates.
Test parameters of key dates in Excel (up to 1900-03-01) moved to test_datetime. py Note that until 2010-10 there was a dedicated module for testing datetime conversions.

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Check public attribute.

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Removed need for each cell to have a SharedDate() instance. Enforce use of calendar in datetime assignments.

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Removed dependency on SharedDate

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Move date handling to separate method.

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