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