Usually, the sentence requires an 85% serve time. If they get the good time, it'll probably be 4-5 days early, off of the 30 days. Inmate release is different in every facility but it is normally in the early morning hours, not a midnight.
https://www.inmateaid.com/ask-the-inmate/if-my-loved-one-gets-30-days-will-he-get-out-early-and-when-you-are-let-out-what-is-the-time-is-it-at-12-the-night-before#answer
Accepted Answer Date Created: February 21,2015