C#时间格式

string aa = DateTime.Now.ToShortDateString();//"2019/9/23"
string bb = DateTime.Now.ToShortTimeString();//"上午 10:21"
string ff = DateTime.Now.ToLongDateString();//2019年9月23日
string gg = DateTime.Now.ToLongTimeString();//上午 10:22:07

int monthDays = DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month);//获取某月在某年中的天数

DateTime From = DateTime.ParseExact(/*tdFrom.Text*/"201909", "yyyyMM", DateTimeFormatInfo.InvariantInfo);//输入的String转Datetime
DateTime End = DateTime.ParseExact(/*tdEnd.Text*/"201909", "yyyyMM", DateTimeFormatInfo.InvariantInfo);

string nian = DateTime.Now.ToString("Y");//2019年9月
string nian1 = DateTime.Now.ToString("y");//2019年9月

string y1 = DateTime.Now.ToString("R");//Mon, 23 Sep 2019 10:31:48 GMT
string y2 = DateTime.Now.ToString("r");//Mon, 23 Sep 2019 10:32:10 GMT

string xingqiji = DateTime.Now.ToString("dddd");// "星期一"
string zhouji = DateTime.Now.ToString("ddd");//  "周一"
string ri = DateTime.Now.ToString("dd");// 23
string ershisi = DateTime.Now.ToString("HH"); //3
string shier = DateTime.Now.ToString("hh");//15

string chang = DateTime.Now.ToString("F");//2019年9月23日 上午 10:34:08
string chang1 = DateTime.Now.ToString("f");//2019年9月23日 上午 10:34
string cahng2 = DateTime.Now.ToString("G");//2019/9/23 上午 10:35:07
string cahng3 = DateTime.Now.ToString("g");//2019/9/23 上午 10:35
string cahng4 = DateTime.Now.ToString("D");//2019年9月23日
string cahng5 = DateTime.Now.ToString("d");//2019/9/23

string now = DateTime.Now.ToString("yyyy/MM/dd HH:mm:ss");//2019/09/23 10:14:54
string a = DateTime.Now.AddYears(20).ToString("yyyy/MM/dd HH:mm:ss");//2039/09/23 10:14:54
string b = DateTime.Now.AddMonths(-2).ToString("yyyy/MM/dd HH:mm:ss");//"2019/07/23 10:14:54"
string c = DateTime.Now.AddDays(-8).ToString("yyyy/MM/dd HH:mm:ss");//"2019/09/15 10:14:54"
string d = DateTime.Now.AddHours(24).ToString("yyyy/MM/dd HH:mm:ss");//"2019/09/24 10:14:54"
string f = DateTime.Now.AddMinutes(1440).ToString("yyyy/MM/dd HH:mm:ss");//2019/09/24 10:14:54
string g = DateTime.Now.AddSeconds(3600).ToString("yyyy/MM/dd HH:mm:ss");//2019/09/23 11:14:54"
string h = DateTime.Now.AddSeconds(3600).ToString("yyyy/MM/dd HH:mm:ss:ffff");//2019/09/23 11:14:54:7992