DATE

DateTime.Now.ToString("yyyyMMddHHmmss"); // case sensitive

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"
Standard DateTime Formatting
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' (*)
Update using c# 6 string interpolation format
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"