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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.

&& Convert Excel serial number to date 
FUNCTION ExcelSerialNumber2Date(tnExcelSerialNumber)
RETURN {^1899/12/30} + tnExcelSerialNumber 
 
&&-----------------------------------------------
&& Example
? ExcelSerialNumber2Date(39278)			&& 2007-07-15

See Also Convert Unix time to datetime