C#獲取當月第一天和最后一天
當月第一天0時0分0秒：

DateTime.Now.AddDays(1 - DateTime.Now.Day).Date

當月最后一天23時59分59秒：

DateTime.Now.AddDays(1 - DateTime.Now.Day).Date.AddMonths(1).AddSeconds(-1)

C#獲取上個月第一天和最后一天
上個月第一天0時0分0秒：

DateTime.Now.AddMonths(-1).Date.AddDays(1 - DateTime.Now.Day);

上個月最后一天23時59分59秒：
DateTime.Parse(DateTime.Now.AddDays(1 - DateTime.Now.Day).ToShortDateString()).AddSeconds(-1).ToString();

當天0時0分0秒：
DateTime start = Convert.ToDateTime(DateTime.Now.ToString("D").ToString());
當天23時59分59秒：
DateTime end = Convert.ToDateTime(DateTime.Now.AddDays(1).ToString("D").ToString()).AddSeconds(-1);

public String DateToStringBeginOrEnd(Date date,Boolean flag) {
 String time = null;
 SimpleDateFormat dateformat1 = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");
 Calendar calendar1 = Calendar.getInstance();
 //獲取某一天的0點0分0秒 或者 23點59分59秒
 if (flag == true) {
  calendar1.setTime(date);
  calendar1.set(calendar1.get(Calendar.YEAR), calendar1.get(Calendar.MONTH), calendar1.get(Calendar.DAY_OF_MONTH),
   0, 0, 0);
  Date beginOfDate = calendar1.getTime();
  time = dateformat1.format(beginOfDate);
  System.out.println(time);
 }else{
  Calendar calendar2 = Calendar.getInstance();
  calendar2.setTime(date);
  calendar1.set(calendar2.get(Calendar.YEAR), calendar2.get(Calendar.MONTH), calendar2.get(Calendar.DAY_OF_MONTH),
   23, 59, 59);
  Date endOfDate = calendar1.getTime();
  time = dateformat1.format(endOfDate);
  System.out.println(time);
 }
 return time;}