I came across the following passage in a Taiwanese short story titled 水鬼學校和失去媽媽的水獺.
I’m having trouble understanding how the bold part of the last sentence works and what exactly it means. Does 上月 mean the same as 上個月? If so, why is a time expression like that (like “last month” in English) not used adverbially but as an object of the verb 有? Or is the sentence saying, literally, this case now has “a previous month” and that’s supposed to be interpreted as meaning it has lasted long enough to extend into a new month?