从Excel读取各种格式的日期转换为目标格式

程序读取excel中日期时，可能遇到各种各样的格式，转换时经常遇到错误，本示例使用C#程序演示如何转换为目标格式：

public static string ToDate(object val)
{
        if (val==null  || string.IsNullOrEmpty(val.ToString()))
        {
            return "0001-01-01";
        }
        string dateStr = val.ToString();

        if (new Regex(@"^\d{4}-\d{2}-\d{2}$").IsMatch(dateStr))
        {
            //常规日期格式
        }
        else if(new Regex(@"^[0-9]{1,7}$").IsMatch(dateStr))
        {
            return DateTime.FromOADate(double.Parse(dateStr)).ToString("yyyy-MM-dd", System.Globalization.DateTimeFormatInfo.InvariantInfo);
        }
        else if (new Regex(@"^\d{8,}$").IsMatch(dateStr))
        {
            return DateTime.ParseExact(dateStr, "yyyyMMdd", null).ToString("yyyy-MM-dd");
        }
        else if (new Regex(@"^\d{1,2}-\d{1,2}月-\d{2,4}$").IsMatch(dateStr))
        {
            dateStr = new Regex("[\u4e00-\u9fff]").Replace(dateStr, "");
            string[] temp = dateStr.Split('-');
            dateStr = $"{temp[2]}-{temp[1]}-{temp[0]}";
        }
        else
        {
            dateStr = new Regex("[\u4e00-\u9fff]").Replace(dateStr, "");
            dateStr = dateStr.Replace('/', '-').Replace('\\', '-').Replace('.', '-');
        }
 
        string[] dataArr = dateStr.Split('-');
        if (dataArr.Length == 0)
        {
            return "0001-01-01";
        }
        string year = dataArr[0];
        string month = dataArr.Length >= 1 ? dataArr[1] : "01";
        string day = dataArr.Length >= 2 ? dataArr[2] : "01";
        string date= $"{year}-{month}-{day}";
        string format = $"{"".PadLeft(year.Length, 'y')}-{"".PadLeft(month.Length, 'M')}-{"".PadLeft(day.Length, 'd')}";

        return DateTime.ParseExact(date, format, null).ToString("yyyy-MM-dd");
}