参见https://msdn.microsoft.com/zh-cn/library/system.datetime%28v=vs.110%29.aspx
DateTime.Date：返回实例的日期
DateTime.Day：返回该月中的第几天
DateTime.Now：返回当前时间，精确到秒
DateTime.Today：返回当前时间，精确到天
以DateTime.Now为例，常用的方法有ToBinary()，ToString()，ToString(String)，ToShortDateString()，ToShortTimeString()
DateTime.Now.ToBinary(); ----返回当前时间，并转换为64位二进制；
DateTime.Now.ToString();---返回当前时间，并转换为字符串；
DateTime.Now.ToShortDateString();---将当前 DateTime 对象的值转换为其等效的短日期字符串表示形式；
DateTime.Now.ToShortTimeString();---将当前 DateTime 对象的值转换为其等效的短时间字符串表示形式;
DateTime.Now.ToString(String);---以指定的日期格式返回当前时间；
MSDN实例程序：
using System; public class DateToStringExample { public static void Main() { DateTime dateValue = new DateTime(2008, 6, 15, 21, 15, 07); // Create an array of standard format strings. string[] standardFmts = {"d", "D", "f", "F", "g", "G", "m", "o", "R", "s", "t", "T", "u", "U", "y"}; // Output date and time using each standard format string. foreach (string standardFmt in standardFmts) Console.WriteLine("{0}: {1}", standardFmt, dateValue.ToString(standardFmt)); Console.WriteLine(); // Create an array of some custom format strings. string[] customFmts = {"h:mm:ss.ff t", "d MMM yyyy", "HH:mm:ss.f", "dd MMM HH:mm:ss", @"\Mon\t\h\: M", "HH:mm:ss.ffffzzz" }; // Output date and time using each custom format string. foreach (string customFmt in customFmts) Console.WriteLine("'{0}': {1}", customFmt, dateValue.ToString(customFmt)); } } // This example displays the following output to the console: // d: 6/15/2008 // D: Sunday, June 15, 2008 // f: Sunday, June 15, 2008 9:15 PM // F: Sunday, June 15, 2008 9:15:07 PM // g: 6/15/2008 9:15 PM // G: 6/15/2008 9:15:07 PM // m: June 15 // o: 2008-06-15T21:15:07.0000000 // R: Sun, 15 Jun 2008 21:15:07 GMT // s: 2008-06-15T21:15:07 // t: 9:15 PM // T: 9:15:07 PM // u: 2008-06-15 21:15:07Z // U: Monday, June 16, 2008 4:15:07 AM // y: June, 2008 // // 'h:mm:ss.ff t': 9:15:07.00 P // 'd MMM yyyy': 15 Jun 2008 // 'HH:mm:ss.f': 21:15:07.0 // 'dd MMM HH:mm:ss': 15 Jun 21:15:07 // '\Mon\t\h\: M': Month: 6 // 'HH:mm:ss.ffffzzz': 21:15:07.0000-07:00
在实际使用时：
string TIME = DateTime.Now.ToString("yyyy-MM-dd");//将日期以2016-03-08的形式输出；
string sqlstr1="delete from T_Origin where datetime<"+"'"+TIME+"'";
string sqlstr2 = "delete from T_Result where datetime<" + "'" + TIME + "'";
Debug.WriteLine(sqlstr1);
程序的输出结果为：
delete fron T_Origin where datetime<2016-03-08;