a month or older; one month behind
Refers to something that is a month past its usual or expected date, such as a late event, a delayed publication, or an item that is a month old.
I accidentally bought a magazine that is a month old.
In some regions, Hinamatsuri is celebrated a month later this year.
back number of a monthly magazine
Specifically used for issues of a monthly publication that are from a previous month, i.e., back issues.
Back issues of the monthly magazine are sometimes discounted at bookstores.