DateTime Format String

DateTime now = new DateTime(2006, 9, 07, 15, 06, 01, 08, DateTimeKind.Local);

now.ToString();      //"09/27/2006 15:06:01"

Year

now.ToString("%y");   //"6"

now.ToString("yy");   //"06"

now.ToString("yyy");  //"2006"

now.ToString("yyyy"); //"2006"

Month

now.ToString("%M");    //"9"

now.ToString("MM");    //"09"

now.ToString("MMM");   //"Sep"

now.ToString("MMMM");  //"September"

Day

now.ToString("%d");    //"7"

now.ToString("dd");    //"07"

now.ToString("ddd");   //"Thu"

now.ToString("dddd");  //"Thursday"

Hour

now.ToString("%h");    //"3"

now.ToString("hh");    //"03"

now.ToString("hhh");   //"03"

now.ToString("hhhh");  //"03"

now.ToString("%H");    //"15"

now.ToString("HH");    //"15"

now.ToString("HHH");   //"15"

now.ToString("HHHH");  //"15"

Minutes

now.ToString("%m");    //"3"

now.ToString("mm");    //"03"

now.ToString("mmm");   //"03"

now.ToString("mmmm");  //"03"

Seconds

now.ToString("%s");    //"1"

now.ToString("ss");    //"01"

now.ToString("sss");   //"01"

now.ToString("ssss");  //"01"

Milliseconds

now.ToString("%f");    //"0"

now.ToString("ff");    //"00"

now.ToString("fff");   //"008"

now.ToString("ffff");  //"0080"

now.ToString("%F");    //""

now.ToString("FF");    //""

now.ToString("FFF");   //"008"

now.ToString("FFFF");  //"008"

Kind

now.ToString("%K");    //"-07:00"

now.ToString("KK");    //"-07:00-07:00"

now.ToString("KKK");   //"-07:00-07:00-07:00"

now.ToString("KKKK");  //"-07:00-07:00-07:00-07:00"

(Source)