Document properties, both built-in and custom, are a Microsoft Office-wide feature. The Word 2003 and Microsoft Office Excel expose document properties and provide the same way to work with them. These properties provide a powerful way to add and track specific information for a document.

Excel stores dates as the number of days since 1900-01-00. The number 1 represents 1900-01-01 and so on. The time part, if present, is represented as decimal portion. There's a bug in implementation of that sequence. It assumes that there was 1900-02-29 but 1900 is not a leap year. As result the code below correctly convert dates starting with 1900-03-01 only.

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If you ever find yourself needing to convert from UNIX time in Excel, here's the solution.