The Bereznikers

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.