DateTime总结

// sql date
select count([Action]), CONVERT(CHAR(10), [Timestamp], 111) as [SpecificPeriod] from UserAction group by CONVERT(CHAR(10), [Timestamp], 111) 


// 时间相减          
DateTime dtone = Convert.ToDateTime("2007-1-1 05:32:22"); DateTime dtwo = Convert.ToDateTime("2007-1-1 04:20:15");  
TimeSpan span = dtone.Subtract(dtwo);    Response.Write(span.Days + "天" + span.Hours + "小时" + span.Minutes + "分钟" + span.Seconds+"秒");

// compare the two date
double unixTime = Convert.ToInt64(codes);                var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);                DateTime dt = epoch.AddSeconds(unixTime);                DateTime now = DateTime.UtcNow;                if (dt > now)                {                    return true;                }
// 得到当周/当月的第一天/Last Four weeks
using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks; namespace ConsoleApplication4{    class Program    {        static void Main(string[] args)        {            DateTime thisWe = GetWeekStart(DateTime.Now, 3);            DateTime lastTh = thisWe.AddDays(-6);            lastTh = Convert.ToDateTime(lastTh.ToString("yyyy-MM-dd 00:00:00"));            thisWe = Convert.ToDateTime(thisWe.ToString("yyyy-MM-dd 23:59:59"));            Console.WriteLine("the wed is : " + thisWe);            Console.WriteLine("the last Th is : " + lastTh);                        Console.ReadKey();        }         /// <summary>         /// 获得当周起始日期 贾世义         /// </summary>         /// <param name="startDate">提供日期</param>         /// <param name="weekStart">周开始(0为周日 1周一 2周二 3周三 4周四 5 周五 6 周六)</param>         /// <returns></returns>         public static DateTime GetWeekStart(DateTime startDate, int weekStart)        {            int day = (int)startDate.DayOfWeek;            if (day > weekStart)            {                return startDate.AddDays(weekStart - day);            }            else if (day < weekStart)            {                return startDate.AddDays(weekStart - day - 7);            }            else            {                return startDate;            }        }   /// <summary>        /// 取得某月的第一天        /// </summary>        /// <param name="datetime">要取得月份第一天的时间</param>        /// <returns></returns>        private DateTime FirstDayOfMonth()        {            DateTime datetime = Convert.ToDateTime(DateTime.Now.ToString("yyyy-MM-dd 00:00:00"));            return datetime.AddDays(1 - datetime.Day);        }         /**/        /// <summary>        /// 取得某月的最后一天        /// </summary>        /// <param name="datetime">要取得月份最后一天的时间</param>        /// <returns></returns>        private DateTime LastDayOfMonth()        {            DateTime datetime = Convert.ToDateTime(DateTime.Now.ToString("yyyy-MM-dd 23:59:59"));            return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);        }    }}
// Microsoft Sql Server 中利用Convert 函数转换Datetime 到 String
// http://blog.csdn.net/forlinux/article/details/2723694