He will have to do 85% (good behavior is credited at the beginning of the sentence and is 15%) of 12 months minus the 31 days credit.
https://www.inmateaid.com/ask-the-inmate/my-husband-got-a-year-sentence-with-31-days-credit-how-long-will-he-actually#answer
Accepted Answer Date Created: November 19,2015