使用Laravel在PHP中按时间戳，后端过滤SQL数据库查询

So I'm trying to build a query to my SQL database from my backend (that's written in php with Laravel). I have a table in my database with a column that's titled "created_at", carrying values of the 'timestamp'-type. What I want to do is, when I send a query to my database, to only pick the rows (or instances, I guess) of my table that are created 30 days or less from the current date. I've tried experimenting back and forth with the 'where' keyword, but I couldn't get it to work as I wanted it to. So I'm trying to build a query to my SQL databa