Question

How do I convert from excel serial date to a .NET date time?

For example 39938 is 05/05/2009.

Answer 1

Where 39938 is the number of days since 1/1/1900?

In that case, use the framework library function DateTime.FromOADate() .
This function encapsulates all the specifics, and does bounds checking.

For its historical value, here is a possible implementation:

(C#)

public static DateTime FromExcelSerialDate(int SerialDate) {     if (SerialDate > 59) SerialDate -= 1; //Excel/Lotus 2/29/1900 bug        return new DateTime(1899, 12, 31).AddDays(SerialDate); } 

VB

Public Shared Function FromExcelSerialDate(ByVal SerialDate As Integer) As DateTime     If SerialDate > 59 Then SerialDate -= 1 ' Excel/Lotus 2/29/1900 bug     Return New DateTime(1899, 12, 31).AddDays(SerialDate) End Function 

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[Update]:
Hmm… A quick test of that shows it’s actually two days off. Not sure where the difference is.

Okay: problem fixed now. See the comments for details.