//默认当年1月1号
ViewBag.time1 = new DateTime(DateTime.Now.Year, 1, 1).ToString("yyyy-MM-dd");
ViewBag.time2 = DateTime.Now.ToString("yyyy-MM-dd");
DateTime dt=DateTime.Now;
int weeknow = Convert.ToInt32(DateTime.Now.DayOfWeek);
int dayspan = (-1) * weeknow + 1;
DateTime dt2=dt.AddMonths(1);
//获取本周第一天
this.Label1.Text = DateTime.Now.AddDays(dayspan).ToString("yyyy-MM-dd");
//本月第一天
this.Label1.Text = dt.AddDays(-(dt.Day) + 1).ToString("yyyy-MM-dd");
//本月最后一天
this.Label1.Text = dt2.AddDays(-dt.Day).ToString("yyyy-MM-dd");
//上个月第一天
this.Label1.Text = dt.AddMonths(-1).AddDays(-dt.Day+1).ToString("yyyy-MM-dd");
//上个月最后一天
this.Label1.Text = dt.AddDays(-dt.Day).ToString("yyyy-MM-dd");
//上一年第一天
this.Label1.Text = dt.AddYears(-1).AddMonths(-dt.Month+1).AddDays(-dt.Day+1).ToString("yyyy-MM-dd");
//上一年最后一天
this.Label1.Text = dt.AddMonths(-dt.Month + 1).AddDays(-dt.Day).ToString("yyyy-MM-dd");
//本年第一天
this.Label1.Text = dt.AddMonths(-dt.Month + 1).AddDays(-dt.Day+1).ToString("yyyy-MM-dd");
//上周第一天
this.Label1.Text = DateTime.Now.AddDays(daydiff-7).ToString("yyyy-MM-dd");
//上周最后一天
this.Label1.Text = DateTime.Now.AddDays(daydiff - 1).ToString("yyyy-MM-dd");
DateTime dt=DateTime.Now;
int weeknow = Convert.ToInt32(DateTime.Now.DayOfWeek);
int dayspan = (-1) * weeknow + 1;
DateTime dt2=dt.AddMonths(1);
//获取本周第一天
this.Label1.Text = DateTime.Now.AddDays(dayspan).ToString("yyyy-MM-dd");
//本月第一天
this.Label1.Text = dt.AddDays(-(dt.Day) + 1).ToString("yyyy-MM-dd");
//本月最后一天
this.Label1.Text = dt2.AddDays(-dt.Day).ToString("yyyy-MM-dd");
//上个月第一天
this.Label1.Text = dt.AddMonths(-1).AddDays(-dt.Day+1).ToString("yyyy-MM-dd");
//上个月最后一天
this.Label1.Text = dt.AddDays(-dt.Day).ToString("yyyy-MM-dd");
//上一年第一天
this.Label1.Text = dt.AddYears(-1).AddMonths(-dt.Month+1).AddDays(-dt.Day+1).ToString("yyyy-MM-dd");
//上一年最后一天
this.Label1.Text = dt.AddMonths(-dt.Month + 1).AddDays(-dt.Day).ToString("yyyy-MM-dd");
//本年第一天
this.Label1.Text = dt.AddMonths(-dt.Month + 1).AddDays(-dt.Day+1).ToString("yyyy-MM-dd");
//上周第一天
this.Label1.Text = DateTime.Now.AddDays(daydiff-7).ToString("yyyy-MM-dd");
//上周最后一天
this.Label1.Text = DateTime.Now.AddDays(daydiff - 1).ToString("yyyy-MM-dd");
AND DATEPART(YEAR,m.selltime) = DATEPART(YEAR,DATEADD(YEAR,-1,GETDATE()))
DATEDIFF(YEAR,m.SellTime,GETDATE())=1 -- 当前时间（年份）减去 字段的年份如果等于 1 表示去年的
//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