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.For more info about Excel dates see How to use dates and times in Excel and Dates And Times In Excel
If you ever find yourself needing to convert from UNIX time in Excel, here's the solution.