你只需要在月(MM)和分钟(MM)之间小心:

DateTime dt = DateTime.Now; // Or whatever
string s = dt.ToString("yyyyMMddHHmmss");

(还要注意，HH是24小时时钟，而HH是12小时时钟，通常与t或tt一起作为am/pm指示符。)

如果你想把它作为复合格式字符串的一部分，你可以使用:

string s = string.Format("The date/time is: {0:yyyyMMddHHmmss}", dt);

有关详细信息，请参阅MSDN页面自定义日期和时间格式。

实际上，您已经自己编写了格式。

你的日期。ToString（“yyyyMMddHHmmss”）

MM =两位数月 Mm =两位数的分钟 HH =两位数小时，24小时钟 Hh =两位数小时，12小时钟

其他一切都应该不言自明。

以上任何一个答案都不能解决你的问题的可能性很小。尽管如此，我还是分享了我的方法，它总是适用于不同格式的约会时间。

//Definition   
     public static DateTime ConvertPlainStringToDatetime(string Date, string inputFormat, string  outputFormat)
            {
                DateTime date;
                CultureInfo enUS = new CultureInfo("en-US");
                DateTime.TryParseExact(Date, inputFormat, enUS,
                                    DateTimeStyles.AdjustToUniversal, out date);

                string formatedDateTime = date.ToString(outputFormat);
                return Convert.ToDateTime(formatedDateTime);   
            }
//Calling

    string oFormat = "yyyy-MM-dd HH:mm:ss";
    DateTime requiredDT = ConvertPlainStringToDatetime("20190205","yyyyMMddHHmmss", oFormat  );
    DateTime requiredDT = ConvertPlainStringToDatetime("20190508-12:46:42","yyyyMMdd-HH:mm:ss", oFormat);

这个网站有很好的例子

// create date time 2008-03-09 16:05:07.123
DateTime dt = new DateTime(2008, 3, 9, 16, 5, 7, 123);

String.Format("{0:y yy yyy yyyy}",      dt);  // "8 08 008 2008"   year
String.Format("{0:M MM MMM MMMM}",      dt);  // "3 03 Mar March"  month
String.Format("{0:d dd ddd dddd}",      dt);  // "9 09 Sun Sunday" day
String.Format("{0:h hh H HH}",          dt);  // "4 04 16 16"      hour 12/24
String.Format("{0:m mm}",               dt);  // "5 05"            minute
String.Format("{0:s ss}",               dt);  // "7 07"            second
String.Format("{0:f ff fff ffff}",      dt);  // "1 12 123 1230"   sec.fraction
String.Format("{0:F FF FFF FFFF}",      dt);  // "1 12 123 123"    without zeroes
String.Format("{0:t tt}",               dt);  // "P PM"            A.M. or P.M.
String.Format("{0:z zz zzz}",           dt);  // "-6 -06 -06:00"   time zone

// month/day numbers without/with leading zeroes
String.Format("{0:M/d/yyyy}",           dt);  // "3/9/2008"
String.Format("{0:MM/dd/yyyy}",         dt);  // "03/09/2008"

// day/month names
String.Format("{0:ddd, MMM d, yyyy}",   dt);  // "Sun, Mar 9, 2008"
String.Format("{0:dddd, MMMM d, yyyy}", dt);  // "Sunday, March 9, 2008"

// two/four digit year
String.Format("{0:MM/dd/yy}",           dt);  // "03/09/08"
String.Format("{0:MM/dd/yyyy}",         dt);  // "03/09/2008"

标准日期时间格式

String.Format("{0:t}", dt);  // "4:05 PM"                           ShortTime
String.Format("{0:d}", dt);  // "3/9/2008"                          ShortDate
String.Format("{0:T}", dt);  // "4:05:07 PM"                        LongTime
String.Format("{0:D}", dt);  // "Sunday, March 09, 2008"            LongDate
String.Format("{0:f}", dt);  // "Sunday, March 09, 2008 4:05 PM"    LongDate+ShortTime
String.Format("{0:F}", dt);  // "Sunday, March 09, 2008 4:05:07 PM" FullDateTime
String.Format("{0:g}", dt);  // "3/9/2008 4:05 PM"                  ShortDate+ShortTime
String.Format("{0:G}", dt);  // "3/9/2008 4:05:07 PM"               ShortDate+LongTime
String.Format("{0:m}", dt);  // "March 09"                          MonthDay
String.Format("{0:y}", dt);  // "March, 2008"                       YearMonth
String.Format("{0:r}", dt);  // "Sun, 09 Mar 2008 16:05:07 GMT"     RFC1123
String.Format("{0:s}", dt);  // "2008-03-09T16:05:07"               SortableDateTime
String.Format("{0:u}", dt);  // "2008-03-09 16:05:07Z"              UniversalSortableDateTime

/*
Specifier   DateTimeFormatInfo property     Pattern value (for en-US culture)
    t           ShortTimePattern                    h:mm tt
    d           ShortDatePattern                    M/d/yyyy
    T           LongTimePattern                     h:mm:ss tt
    D           LongDatePattern                     dddd, MMMM dd, yyyy
    f           (combination of D and t)            dddd, MMMM dd, yyyy h:mm tt
    F           FullDateTimePattern                 dddd, MMMM dd, yyyy h:mm:ss tt
    g           (combination of d and t)            M/d/yyyy h:mm tt
    G           (combination of d and T)            M/d/yyyy h:mm:ss tt
    m, M        MonthDayPattern                     MMMM dd
    y, Y        YearMonthPattern                    MMMM, yyyy
    r, R        RFC1123Pattern                      ddd, dd MMM yyyy HH':'mm':'ss 'GMT' (*)
    s           SortableDateTi­mePattern             yyyy'-'MM'-'dd'T'HH':'mm':'ss (*)
    u           UniversalSorta­bleDateTimePat­tern    yyyy'-'MM'-'dd HH':'mm':'ss'Z' (*)
                                                    (*) = culture independent   
*/

更新 使用c# 6字符串插值格式

// create date time 2008-03-09 16:05:07.123
DateTime dt = new DateTime(2008, 3, 9, 16, 5, 7, 123);

$"{dt:y yy yyy yyyy}";  // "8 08 008 2008"   year
$"{dt:M MM MMM MMMM}";  // "3 03 Mar March"  month
$"{dt:d dd ddd dddd}";  // "9 09 Sun Sunday" day
$"{dt:h hh H HH}";      // "4 04 16 16"      hour 12/24
$"{dt:m mm}";           // "5 05"            minute
$"{dt:s ss}";           // "7 07"            second
$"{dt:f ff fff ffff}";  // "1 12 123 1230"   sec.fraction
$"{dt:F FF FFF FFFF}";  // "1 12 123 123"    without zeroes
$"{dt:t tt}";           // "P PM"            A.M. or P.M.
$"{dt:z zz zzz}";       // "-6 -06 -06:00"   time zone

// month/day numbers without/with leading zeroes
$"{dt:M/d/yyyy}";    // "3/9/2008"
$"{dt:MM/dd/yyyy}";  // "03/09/2008"

// day/month names
$"{dt:ddd, MMM d, yyyy}";    // "Sun, Mar 9, 2008"
$"{dt:dddd, MMMM d, yyyy}";  // "Sunday, March 9, 2008"

// two/four digit year
$"{dt:MM/dd/yy}";    // "03/09/08"
$"{dt:MM/dd/yyyy}";  // "03/09/2008"

指定格式化的DateTime为Utc:

步骤1 -初始日期

var initialDtm = DateTime.Now;

步骤2 -格式日期为愿意("yyyyMMddHHmmss")

var formattedDtm = DateTime.ParseExact(initialDtm.ToString("yyyyMMddHHmmss"), "yyyyMMddHHmmss", CultureInfo.InvariantCulture);    

步骤3 -指定日期类型(Utc)

var specifiedDtm = DateTime.SpecifyKind(formattedDtm, DateTimeKind.Utc);

在.Net Standard 2中，你可以像下面这样格式化DateTime:

DateTime dt = DateTime.Now;
CultureInfo iv = CultureInfo.InvariantCulture;

// Default formats
// D - long date           Tuesday, 24 April 2018
// d - short date          04/24/2018
// F - full date long      Tuesday, 24 April 2018 06:30:00
// f - full date short     Tuesday, 24 April 2018 06:30
// G - general long        04/24/2018 06:30:00
// g - general short       04/24/2018 06:30
// U - universal full      Tuesday, 24 April 2018 06:30:00
// u - universal sortable  2018-04-24 06:30:00
// s - sortable            2018-04-24T06:30:00
// T - long time           06:30:00
// t - short time          06:30
// O - ISO 8601            2018-04-24T06:30:00.0000000
// R - RFC 1123            Tue, 24 Apr 2018 06:30:00 GMT           
// M - month               April 24
// Y - year month          2018 April
Console.WriteLine(dt.ToString("D", iv));

// Custom formats
// M/d/yy                  4/8/18
// MM/dd/yyyy              04/08/2018
// yy-MM-dd                08-04-18
// yy-MMM-dd ddd           08-Apr-18 Sun
// yyyy-M-d dddd           2018-4-8 Sunday
// yyyy MMMM dd            2018 April 08      
// h:mm:ss tt zzz          4:03:05 PM -03
// HH:m:s tt zzz           16:03:05 -03:00
// hh:mm:ss t z            04:03:05 P -03
// HH:mm:ss tt zz          16:03:05 PM -03      
Console.WriteLine(dt.ToString("M/d/yy", iv));