DateTime dt = DateTime.Now;
//本月第一天時間
DateTime dt_First = dt.AddDays(-(dt.Day) + 1);
Label1.Text = dt_First.ToString("yyyy-MM-dd");
//將本月月數+1
DateTime dt2 = dt.AddMonths(1);
//本月最后一天時間
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");