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BREAKING CHANGE! In previous versions of PHPExcel up to and including 1.6.6,
when a cell had a date-like number format code, it was possible to enter a date
directly using an integer PHP-time without converting to Excel date format.
Starting with PHPExcel 1.6.7 this is no longer supported. Refer to the developer
documentation for more information on entering dates into a cell.
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