It's about time in this case, per diem is an adverb meaning By the day and is usually used after the person or thing it modifies. Per is employed as a preposition in this case meaning 'for or in each or every' and used to is to separate the word in the expression. Latin, literally per day, "daily" would be a good synonym. For reckoned on a daily basis.

The phrase per diem is used primarily in the business world such as a daily allowance for expenses;
i.e.The traveling salesman was given a fixed per diem.

Also used in legal arenas, and to some extent as a medical term to take prescribed amounts of medications a specific number of times per diem, or per day.