30 days listing expired date has been added automatically, if you leave it blank

every time when i post a coupon, a 30 day list expire has been set by system. I have delete all the package and membership. I don’t want any list to expire. this is bug or feature? a default 30 day expire date? i think it is like to be a bug. Right now I have to edit every single post twice to get rid of expire date. any way to fix it?