string mytime = str.Trim();
                    IFormatProvider culture = new CultureInfo("zh-CN", true);
                    string[] expectedFormats = { "dd-M月-yyyy" };
                    DateTime dt;
                    if (DateTime.TryParseExact(mytime, expectedFormats, culture, 
                    DateTimeStyles.AllowInnerWhite, out dt))
                    {
                        mytime = dt.ToString("yyyy-MM-dd");
                    }
                    else
                    {
                        mytime = str.Trim();//格式转换失败，返回原本的值
                    }

参考链接：https://q.cnblogs.com/q/94907/