根据当前的日期获取到周起始，月起始，年起始

一、根据当前的日期获取到周起始和截止

 1 /// 

 2 /// 得到本周第一天(以星期天为第一天)
 3 /// 
 4 /// 
 5 /// 
 6 public DateTime GetWeekFirstDaySun(DateTime datetime)
 7 {
 8     //星期天为第一天
 9     int weeknow = Convert.ToInt32(datetime.DayOfWeek);
10     int daydiff = (-1) * weeknow;
11 
12     //本周第一天
13     string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
14     return Convert.ToDateTime(FirstDay);
15 }

 1 /// 

 2 /// 得到本周第一天(以星期一为第一天)
 3 /// 
 4 /// 
 5 /// 
 6 public DateTime GetWeekFirstDayMon(DateTime datetime)
 7 {
 8     //星期一为第一天
 9     int weeknow = Convert.ToInt32(datetime.DayOfWeek);
10 
11     //因为是以星期一为第一天，所以要判断weeknow等于0时，要向前推6天。
12     weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));
13     int daydiff = (-1) * weeknow;
14 
15     //本周第一天
16     string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
17     return Convert.ToDateTime(FirstDay);
18 }

 1 /// 

 2 /// 得到本周最后一天(以星期六为最后一天)
 3 /// 
 4 /// 
 5 /// 
 6 public DateTime GetWeekLastDaySat(DateTime datetime)
 7 {
 8     //星期六为最后一天
 9     int weeknow = Convert.ToInt32(datetime.DayOfWeek);
10     int daydiff = (7 - weeknow) - 1;
11 
12     //本周最后一天
13     string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
14     return Convert.ToDateTime(LastDay);
15 }

 1 /// 

 2 /// 得到本周最后一天(以星期天为最后一天)
 3 /// 
 4 /// 
 5 /// 
 6 public DateTime GetWeekLastDaySun(DateTime datetime)
 7 {
 8     //星期天为最后一天
 9     int weeknow = Convert.ToInt32(datetime.DayOfWeek);
10     weeknow = (weeknow == 0 ? 7 : weeknow);
11     int daydiff = (7 - weeknow);
12 
13     //本周最后一天
14     string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
15     return Convert.ToDateTime(LastDay);
16 }

二、根据当前的日期获取到月起始和截止

int year = DateTime.Now.Year;
int month = DateTime.Now.Month;

DateTime firstDayOfThisMonth = new DateTime(year, month, 1);
DateTime lastDayOfThisMonth = new DateTime(year, month, DateTime.DaysInMonth(year, month));

     SQL语句实现

 1 DECLARE @NOW DATETIME
 2 DECLARE @FistDayOfThisMonth DATETIME  
 3 DECLARE @LastDayOfThisMonth DATETIME  
 4 
 5 SET @NOW = GETDATE()
 6 SET @FistDayOfThisMonth = @NOW - DAY(@NOW) + 1  
 7 SET @LastDayOfThisMonth = DATEADD(MONTH, 1, @NOW - DAY(@NOW) + 1) - 1
 8 
 9 SELECT @FistDayOfThisMonth
10 SELECT @LastDayOfThisMonth

三、根据当前的日期获取到年起始和截止

1 int year = DateTime.Now.Year;
2 int month = DateTime.Now.Month;
3 
4 DateTime firstDayOfThisMonth = new DateTime(year, 1, 1);
5 DateTime lastDayOfThisMonth = new DateTime(year, 12, DateTime.DaysInMonth(year, 12));

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