int year = DateTime.Now.Year;//当前年  int mouth = DateTime.Now.Month;//当前月    int beforeYear = 0;  int beforeMouth = 0;  if (mouth <= 1)//如果当前月是一月，那么年份就要减1  {      beforeYear = year - 1;      beforeMouth =12;//上个月  }  else  {     beforeYear = year;     beforeMouth = mouth - 1;//上个月  }  string beforeMouthOneDay = beforeYear + "年" + beforeMouth + "月" + 1 + "日";//上个月第一天  string beforeMouthLastDay = beforeYear + "年" + beforeMouth + "月" + DateTime.DaysInMonth(year, beforeMouth) + "日";//上个月最后一天

string beforeMouthLastDay = DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddDays(-1).ToString("yyyy-MM-dd"); //获取上个月最后一天日期