oracle计算日期的问题！

写了一个包，包里有一个存储过程
存储过程有一个传入参数和一个游标
功能：根据传入的日期，计算上一个月的日期，然后查询出这两个月之间的全部信息
但是存储过程报错了，各位帮我看看，谢谢

create or replace package body PKG_T_BlogSearchDate
as
procedure PRO_T_BlogSearchDate(bgdate varchar2,mycur_out out mycur)
is
mm_temp date;--上一月
begin
select to_char(add_months(trunc(bgdate),-1),'yyyy-mm') into mm_temp from dual; --根据传入月份，获取上一月份
open mycur_out for
select * from t_blog where bg_date between mm_temp and bgdate order by bg_date desc;
end PRO_T_BlogSearchDate;
end PKG_T_BlogSearchDate;

错误信息：ora-00932不一致的数据类型：要求date得到却是number

---

1)
--报错在这里：
select to_char(add_months(trunc(bgdate),-1),'yyyy-mm') into mm_temp from dual; --根据传入月份，获取上一月份

改成：
select add_months(to_date(bgdate || '-01', 'yyyy-mm-dd'), -1) into mm_temp from dual;

2)
select * from t_blog where bg_date between mm_temp and bgdate order by bg_date desc;
这里也有问题，bg_date字段是什么类型的？

---

create or replace package body PKG_T_BlogSearchDate
as
procedure PRO_T_BlogSearchDate(bgdate varchar2,mycur_out out mycur)
is
mm_temp date;--上一月
begin
select to_char(add_months(trunc(bgdate),-1),'yyyy-mm') into mm_temp from dual; --根据传入月份，获取上一月份
open mycur_out for
select * from