"Monthly" and other time-related words ending in -ly are commonly used as an adjective and noun. It might be nice to have them allowed without needing to resort to turning off the adverb rule.

eg: One does not back up monthly as in the style that a month would back up.

One can do a monthly backup.

I could informally refer to such a backup as a "monthly", but that may be mistaken for something else entirely.