by the day
Also, by the hour or week or month or year. According to a specific time period, as in I'm renting this car by the day, or He's being paid by the hour. This usage generally describes some kind of rate. [1400s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.