How to insert a date to an Open XML worksheet?

I used the code provided by Andrew J, but the DataType CellValues.Date produced a corrupted xlsx-file for me.

The DataType CellValues.Number worked fine for me (Don't forget to set NumberFormatId):

cell.DataType = new EnumValue<CellValues>(CellValues.Number);

My whole code:

DateTime valueDate = DateTime.Now;
string valueString = valueDate.ToOADate().ToString();
CellValue cellValue = new CellValue(valueString);

Cell cell = new Cell();
cell.DataType = new EnumValue<CellValues>(CellValues.Number);
cell.StyleIndex = yourStyle; //StyleIndex of CellFormat cfBaseDate -> See below
cell.Append(cellValue);

My CellFormat for this cell in the Stylesheet looks like:

CellFormat cfBaseDate = new CellFormat() { 
 ApplyNumberFormat = true,
 NumberFormatId = 14, //14 is a localized short Date (d/m/yyyy) -> See list below
 //Some further styling parameters
}; 

If you'd like to format your date another way, here is a list of all default Excel NumberFormatId's

ID  FORMAT CODE
0   General
1   0
2   0.00
3   #,##0
4   #,##0.00
9   0%
10  0.00%
11  0.00E+00
12  # ?/?
13  # ??/??
14  d/m/yyyy
15  d-mmm-yy
16  d-mmm
17  mmm-yy
18  h:mm tt
19  h:mm:ss tt
20  H:mm
21  H:mm:ss
22  m/d/yyyy H:mm
37  #,##0 ;(#,##0)
38  #,##0 ;[Red](#,##0)
39  #,##0.00;(#,##0.00)
40  #,##0.00;[Red](#,##0.00)
45  mm:ss
46  [h]:mm:ss
47  mmss.0
48  ##0.0E+0
49  @

Source of list: https://github.com/ClosedXML/ClosedXML/wiki/NumberFormatId-Lookup-Table

I know this list is from ClosedXML, but it's the same in OpenXML.

There are 2 ways to store dates in OpenXml; by writing a number (using ToOADate) and setting the DataType to Number or by writing an ISO 8601 formatted date and setting the DataType to Date. Note that the default DataType is Number so if you go with the first option you don't have to set the DataType.

Whichever method you choose, you'll need to set the style as Excel displays both methods identically. The following code shows an example of writing a date using the Number format (with and without explicitly setting the DataType) and using the ISO 8601 format.

using (SpreadsheetDocument document = SpreadsheetDocument.Create(filename, SpreadsheetDocumentType.Workbook))
{
    //fluff to generate the workbook etc
    WorkbookPart workbookPart = document.AddWorkbookPart();
    workbookPart.Workbook = new Workbook();

    var worksheetPart = workbookPart.AddNewPart<WorksheetPart>();
    worksheetPart.Worksheet = new Worksheet();

    Sheets sheets = workbookPart.Workbook.AppendChild(new Sheets());

    Sheet sheet = new Sheet() { Id = workbookPart.GetIdOfPart(worksheetPart), SheetId = 1, Name = "Sheet" };
    sheets.Append(sheet);

    workbookPart.Workbook.Save();

    var sheetData = worksheetPart.Worksheet.AppendChild(new SheetData());

    //add the style
    Stylesheet styleSheet = new Stylesheet();

    CellFormat cf = new CellFormat();
    cf.NumberFormatId = 14;
    cf.ApplyNumberFormat = true;

    CellFormats cfs = new CellFormats();
    cfs.Append(cf);
    styleSheet.CellFormats = cfs;

    styleSheet.Borders = new Borders();
    styleSheet.Borders.Append(new Border());
    styleSheet.Fills = new Fills();
    styleSheet.Fills.Append(new Fill());
    styleSheet.Fonts = new Fonts();
    styleSheet.Fonts.Append(new Font());

    workbookPart.AddNewPart<WorkbookStylesPart>();
    workbookPart.WorkbookStylesPart.Stylesheet = styleSheet;

    CellStyles css = new CellStyles();
    CellStyle cs = new CellStyle();
    cs.FormatId = 0;
    cs.BuiltinId = 0;
    css.Append(cs);
    css.Count = UInt32Value.FromUInt32((uint)css.ChildElements.Count);
    styleSheet.Append(css);

    Row row = new Row();

    DateTime date = new DateTime(2017, 6, 24);

    /*** Date code here ***/
    //write an OADate with type of Number
    Cell cell1 = new Cell();
    cell1.CellReference = "A1";
    cell1.CellValue = new CellValue(date.ToOADate().ToString());
    cell1.DataType = new EnumValue<CellValues>(CellValues.Number);
    cell1.StyleIndex = 0;
    row.Append(cell1);

    //write an OADate with no type (defaults to Number)
    Cell cell2 = new Cell();
    cell2.CellReference = "B1";
    cell2.CellValue = new CellValue(date.ToOADate().ToString());
    cell1.StyleIndex = 0;
    row.Append(cell2);

    //write an ISO 8601 date with type of Date
    Cell cell3 = new Cell();
    cell3.CellReference = "C1";
    cell3.CellValue = new CellValue(date.ToString("yyyy-MM-dd"));
    cell3.DataType = new EnumValue<CellValues>(CellValues.Date);
    cell1.StyleIndex = 0;
    row.Append(cell3);

    sheetData.AppendChild(row);

    worksheetPart.Worksheet.Save();
}

You have to convert DateTime to double using function ToOADate i.e.:

DateTime dtValue = DateTime.Now;
string strValue = dtValue.ToOADate().ToString(CultureInfo.InvariantCulture);

then set it as CellValue

Cell cell;
cell.DataType = new EnumValue<CellValues>(CellValues.Date);
cell.CellValue = new CellValue(strValue);

Remember to format cell using DateTime formatting, otherwise you will see double value, not date.

When creating new SpreadsheetDocument from scratch, for Date formatting to work, minimal Stylesheet has to be created.

Critical are those few lines:

new CellFormat
{
    NumberFormatId = 14,
    ApplyNumberFormat = true
})

Full Stylesheet class:

using (var spreadSheet = SpreadsheetDocument.Create(ms, SpreadsheetDocumentType.Workbook))
{
    // Workbook
    var workbookPart = spreadSheet.AddWorkbookPart();
    workbookPart.Workbook =
        new Workbook(new Sheets(new Sheet { Name = "Sheet1", SheetId = (UInt32Value) 1U, Id = "rId1" }));

    // Add minimal Stylesheet
    var stylesPart = spreadSheet.WorkbookPart.AddNewPart<WorkbookStylesPart>();
    stylesPart.Stylesheet = new Stylesheet
    {
        Fonts = new Fonts(new Font()),
        Fills = new Fills(new Fill()),
        Borders = new Borders(new Border()),
        CellStyleFormats = new CellStyleFormats(new CellFormat()),
        CellFormats =
            new CellFormats(
                new CellFormat(),
                new CellFormat
                {
                    NumberFormatId = 14,
                    ApplyNumberFormat = true
                })
    };

    // Continue creating `WorksheetPart`...

After Stylesheet is added, DateTime can be formatted:

if (valueType == typeof(DateTime))
{
    DateTime date = (DateTime)value;
    cell.CellValue = new CellValue(date.ToOADate().ToString(CultureInfo.InvariantCulture));

    // "StyleIndex" is "1", because "NumberFormatId=14"
    // is in the 2nd item of `CellFormats` array.
    cell.StyleIndex = 1; 
}

Note that StyleIndex value depends on the order of CellFormat items in the CellFormats array or the Stylesheet object. In this example NumberFormatId = 14 item on the 2nd item in the array.