根据当前系统日期求出本周，本月，本年，上周，上月，上年的第一天

 //今天 DateTime.Now.Date.ToShortDateString();

//昨天，就是今天的日期减一

 DateTime.Now.AddDays(-1).ToShortDateString();

//明天，同理，加一 DateTime.Now.AddDays(1).ToShortDateString();/

/本周(要知道本周的第一天就得先知道今天是星期几，从而得知本周的第一天就是几天前的那一天，要注意的是这里的每一周是从周日始至周六止 DateTime.Now.AddDays(Convert.ToDouble((0-Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString(); DateTime.Now.AddDays(Convert.ToDouble((6-Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString();

//如果你还不明白，再看一下中文显示星期几的方法就应该懂了

//由于DayOfWeek返回的是数字的星期几，我们要把它转换成汉字方便我们阅读，有些人可能会用switch来一个一个地对照，其实不用那么麻烦的

string[] Day= newstring[] { "星期日","星期一","星期二","星期三","星期四","星期五","星期六" }; Day[Convert.ToInt16(DateTime.Now.DayOfWeek)];/

/上周，同理，一个周是7天，上周就是本周再减去7天，下周也是一样

DateTime.Now.AddDays(Convert.ToDouble((0- Convert.ToInt16(DateTime.Now.DayOfWeek)))-7).ToShortDateString(); DateTime.Now.AddDays(Convert.ToDouble((6-Convert.ToInt16(DateTime.Now.DayOfWeek)))- 7).ToShortDateString();

//下周 DateTime.Now.AddDays(Convert.ToDouble((0- Convert.ToInt16(DateTime.Now.DayOfWeek)))+7).ToShortDateString(); DateTime.Now.AddDays(Convert.ToDouble((6-Convert.ToInt16(DateTime.Now.DayOfWeek)))+ 7).ToShortDateString();

//本月,很多人都会说本月的第一天嘛肯定是1号，最后一天就是下个月一号再减一天。当然这是对的

 //一般的写法

 DateTime.Now.Year.ToString()+ DateTime.Now.Month.ToString()+ "1";

//第一天 DateTime.Parse(DateTime.Now.Year.ToString()+ DateTime.Now.Month.ToString()+"1").AddMonths(1).AddDays(-1).ToShortDateString();

//最后一天 //巧用C#里ToString的字符格式化更简便

DateTime.Now.ToString("yyyy-MM-01"); DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).AddDays(-1).ToShortDateString();

 //上个月，减去一个月份

DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(-1).ToShortDateString(); DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

//下个月，加去一个月份

DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).ToShortDateString(); DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(2).AddDays(-1).ToShortDateString();

//7天后 DateTime.Now.Date.ToShortDateString(); DateTime.Now.AddDays(7).ToShortDateString();

//7天前 DateTime.Now.AddDays(-7).ToShortDateString(); DateTime.Now.Date.ToShortDateString();

//本年度，用ToString的字符格式化我们也很容易地算出本年度的第一天和最后一天

DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).ToShortDateString(); DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).AddDays(-1).ToShortDateString();

//上年度，不用再解释了吧

DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(-1).ToShortDateString(); DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddDays(-1).ToShortDateString();

//下年度

DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).ToShortDateString(); DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(2).AddDays(-1).ToShortDateString();

//本季度，很多人都会觉得这里难点，需要写个长长的过程来判断。其实不用的，我们都知道一年四个季度，一个季度三个月

//首先我们先把日期推到本季度第一个月，然后这个月的第一天就是本季度的第一天了

 DateTime.Now.AddMonths(0- ((DateTime.Now.Month- 1)% 3)).ToString("yyyy-MM-01");

//同理，本季度的最后一天就是下季度的第一天减一

DateTime.Parse(DateTime.Now.AddMonths(3- ((DateTime.Now.Month- 1)% 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

//下季度，相信你们都知道了。。。。收工

DateTime.Now.AddMonths(3- ((DateTime.Now.Month- 1)% 3)).ToString("yyyy-MM-01"); DateTime.Parse(DateTime.Now.AddMonths(6- ((DateTime.Now.Month- 1)% 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

 //上季度

DateTime.Now.AddMonths(-3- ((DateTime.Now.Month- 1)% 3)).ToString("yyyy-MM-01"); DateTime.Parse(DateTime.Now.AddMonths(0- ((DateTime.Now.Month- 1)% 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

//定义变量 

        private static DateTime currentDateTime;        //当前时间
        private static DateTime lastDateTime;           //昨天时间
        private static DateTime nextDateTime;           //明天时间

        private static int currentDayOfWeek;            //今天星期几
        private static int lastDayOfWeek;               //昨天星期几
        private static int nextDayOfWeek;               //明天星期几

        private static DateTime currentStartWeek;       //本周周一
        private static DateTime currentEndWeek;         //本周周日
        private static DateTime currentStartMonth;      //本月月初
        private static DateTime currentEndMonth;        //本月月末
        private static DateTime currentStartQuarter;    //本季度初
        private static DateTime currentEndQuarter;      //本季度末
        private static DateTime currentStartYear;       //今年年初
        private static DateTime currentEndYear;         //今年年末

        private static DateTime lastStartWeek;          //上周周一
        private static DateTime lastEndWeek;            //上周周日
        private static DateTime lastStartMonth;         //上月月初
        private static DateTime lastEndMonth;           //上月月末
        private static DateTime lastStartQuarter;       //上季度初
        private static DateTime lastEndQuarter;         //上季度末
        private static DateTime lastStartYear;          //去年年初
        private static DateTime lastEndYear;            //去年年末

        private static DateTime nextStartWeek;          //下周周一
        private static DateTime nextEndWeek;            //下周周日
        private static DateTime nextStartMonth;         //下月月初
        private static DateTime nextEndMonth;           //下月月末
        private static DateTime nextStartQuarter;       //下季度初
        private static DateTime nextEndQuarter;         //下季度末
        private static DateTime nextStartYear;          //明年年初
        private static DateTime nextEndYear;            //明年年末

//计算变量

            currentDateTime = DateTime.Now;                 //当前时间
            lastDateTime = DateTime.Now.AddDays(-1);        //昨天时间
            nextDateTime = DateTime.Now.AddDays(1);         //明天时间

            currentDayOfWeek = Convert.ToInt32(currentDateTime.DayOfWeek.ToString("d"));    //今天星期几
            currentDayOfWeek = Convert.ToInt32(lastDateTime.DayOfWeek.ToString("d"));       //昨天星期几
            currentDayOfWeek = Convert.ToInt32(nextDateTime.DayOfWeek.ToString("d"));       //明天星期几

            currentStartWeek = currentDateTime.AddDays(1 - ((currentDayOfWeek == 0) ? 7 : currentDayOfWeek));   //本周周一
            currentEndWeek = currentStartWeek.AddDays(6);                                                       //本周周日           
            lastStartWeek = currentStartWeek.AddDays(-7);       //上周周一
            lastEndWeek = currentEndWeek.AddDays(-7);           //上周周日
            nextStartWeek = currentStartWeek.AddDays(7);        //下周周一
            nextEndWeek = currentEndWeek.AddDays(7);            //下周周日

            currentStartMonth = currentDateTime.AddDays(1 - currentDateTime.Day);   //本月月初
            currentEndMonth = currentStartMonth.AddMonths(1).AddDays(-1);           //本月月末
            lastStartMonth = currentStartMonth.AddMonths(-1);       //上月月初
            lastEndMonth = currentStartMonth.AddDays(-1);           //上月月末
            nextStartMonth = currentEndMonth.AddDays(1);            //下月月初
            nextEndMonth = nextStartMonth.AddMonths(1).AddDays(-1); //下月月末

            currentStartQuarter = currentDateTime.AddMonths(0 - (currentDateTime.Month - 1) % 3).AddDays(1 - currentDateTime.Day);  //本季度初
            currentEndQuarter = currentStartQuarter.AddMonths(3).AddDays(-1);                                                      //本季度末
            lastStartQuarter = currentStartQuarter.AddMonths(-3);           //上季度初
            lastEndQuarter = currentStartQuarter.AddDays(-1);               //上季度末
            nextStartQuarter = currentEndQuarter.AddDays(1);                //下季度初
            nextEndQuarter = nextStartQuarter.AddMonths(3).AddDays(-1);     //下季度末

            //年度运算
            currentStartYear = new DateTime(currentDateTime.Year, 1, 1);  //今年年初
            currentEndYear = new DateTime(currentDateTime.Year, 12, 31);  //今年年末
            lastStartYear = currentStartYear.AddYears(-1);  //去年年初
            lastEndYear = currentEndYear.AddYears(-1);      //去年年末
            nextStartYear = currentStartYear.AddYears(1);   //明年年初
            nextEndYear = currentEndYear.AddYears(1);       //明年年末

        /// <summary>
        /// 返回星期几字符串函数
        /// </summary>
        /// <param name="myDateTime"></param>
        /// <returns></returns>

        private string GetWeekDayName(DateTime myDateTime)
        {
            string week = "";
            //获取当前日期是星期几
            string dt = myDateTime.DayOfWeek.ToString();
            //根据取得的星期英文单词返回汉字
            switch (dt)
            {
                case "Monday":
                    week = "星期一";
                    break;
                case "Tuesday":
                    week = "星期二";
                    break;
                case "Wednesday":
                    week = "星期三";
                    break;
                case "Thursday":
                    week = "星期四";
                    break;
                case "Friday":
                    week = "星期五";
                    break;
                case "Saturday":
                    week = "星期六";
                    break;
                case "Sunday":
                    week = "星期日";
                    break;
            }
            return week;
        }

//今天   
DateTime.Now.Date.ToShortDateString();   
//昨天，就是今天的日期减一   
DateTime.Now.AddDays(-1).ToShortDateString();   
//明天，同理，加一   
DateTime.Now.AddDays(1).ToShortDateString();   
  
//本周(要知道本周的第一天就得先知道今天是星期几，从而得知本周的第一天就是几天前的那一天，要注意的是这里的每一周是从周日始至周六止   
DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString();   
DateTime.Now.AddDays(Convert.ToDouble((6 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString();   
//如果你还不明白，再看一下中文显示星期几的方法就应该懂了   
//由于DayOfWeek返回的是数字的星期几，我们要把它转换成汉字方便我们阅读，有些人可能会用switch来一个一个地对照，其实不用那么麻烦的                 
string[] Day = new string[] { "星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六" };   
Day[Convert.ToInt16(DateTime.Now.DayOfWeek)];   
  
//上周，同理，一个周是7天，上周就是本周再减去7天，下周也是一样   
DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek))) - 7).ToShortDateString();   
DateTime.Now.AddDays(Convert.ToDouble((6 - Convert.ToInt16(DateTime.Now.DayOfWeek))) - 7).ToShortDateString();   
//下周   
DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek))) + 7).ToShortDateString();   
DateTime.Now.AddDays(Convert.ToDouble((6 - Convert.ToInt16(DateTime.Now.DayOfWeek))) + 7).ToShortDateString();   
//本月,很多人都会说本月的第一天嘛肯定是1号，最后一天就是下个月一号再减一天。当然这是对的   
//一般的写法   
DateTime.Now.Year.ToString() + DateTime.Now.Month.ToString() + "1"; //第一天   
DateTime.Parse(DateTime.Now.Year.ToString() + DateTime.Now.Month.ToString() + "1").AddMonths(1).AddDays(-1).ToShortDateString();//最后一天   
  
//巧用C#里ToString的字符格式化更简便   
DateTime.Now.ToString("yyyy-MM-01");   
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).AddDays(-1).ToShortDateString();   
  
//上个月，减去一个月份   
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(-1).ToShortDateString();   
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();   
//下个月，加去一个月份   
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).ToShortDateString();   
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(2).AddDays(-1).ToShortDateString();   
//7天后   
DateTime.Now.Date.ToShortDateString();   
DateTime.Now.AddDays(7).ToShortDateString();   
//7天前   
DateTime.Now.AddDays(-7).ToShortDateString();   
DateTime.Now.Date.ToShortDateString();   
  
//本年度，用ToString的字符格式化我们也很容易地算出本年度的第一天和最后一天   
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).ToShortDateString();   
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).AddDays(-1).ToShortDateString();   
//上年度，不用再解释了吧   
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(-1).ToShortDateString();   
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddDays(-1).ToShortDateString();   
//下年度   
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).ToShortDateString();   
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(2).AddDays(-1).ToShortDateString();   
  
//本季度，很多人都会觉得这里难点，需要写个长长的过程来判断。其实不用的，我们都知道一年四个季度，一个季度三个月   
//首先我们先把日期推到本季度第一个月，然后这个月的第一天就是本季度的第一天了   
DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");   
//同理，本季度的最后一天就是下季度的第一天减一   
DateTime.Parse(DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();   
//下季度，相信你们都知道了。。。。收工   
DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");   
DateTime.Parse(DateTime.Now.AddMonths(6 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();   
//上季度   
DateTime.Now.AddMonths(-3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");   
DateTime.Parse(DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();  

原文：http://blog.csdn.net/hefeng_aspnet/article/details/6616650