.Net中获取一周第一天、最后一天，一月第一天、最后一天

        DateTime dt = DateTime.Now;
        //本月第一天时间    
        DateTime dt_First = dt.AddDays(-(dt.Day) + 1);
        Label1.Text = dt_First.ToString("yyyy-MM-dd");
        //将本月月数+1  
        DateTime dt2 = dt.AddMonths(1);
        //本月最后一天时间  
        DateTime dt_Last = dt2.AddDays(-(dt.Day));
        Label2.Text = dt_Last.ToString("yyyy-MM-dd"); 
        int weeknow = Convert.ToInt32(System.DateTime.Now.DayOfWeek);
        int daydiff = (-1) * weeknow + 1;
        int dayadd = 7 - weeknow;

　　 //本周第一天
        lblBegin.Text = System.DateTime.Now.AddDays(daydiff).ToString("yyyy-MM-dd");

       //本周最后一天
        lblEnd.Text = System.DateTime.Now.AddDays(dayadd).ToString("yyyy-MM-dd");