The date values are stored in the Worksheet as number values with specific number format. This is the reason why when you get the value of the cell the result is number. If you want to get the formatted result, you could use the following code snippet:
CellSelection cellSelection =
ICellValue value = cellSelection.GetValue().Value;
CellValueFormat format = cellSelection.GetFormat().Value;
CellValueFormatResult formatResult = format.GetFormatResult(value);
result = formatResult.InfosText;
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