Lendo uma data do XLSX usando o Open XML SDK
public static bool TryParseExcelDateTime(string excelDateTimeAsString, out DateTime dateTime)
{
double oaDateAsDouble;
if (!double.TryParse(excelDateTimeAsString, out oaDateAsDouble)) //this line is Culture dependent!
return false;
//[...]
dateTime = DateTime.FromOADate(oaDateAsDouble);
Code with bulbul