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