In a perfect world, the person would serve one year. But there are factors that might contribute to the inmate being released early. So they may not serve a full year sentence. Good behavior is one of the factors.
Some times it depends on the severity of the crime. Any way if the inmate is healthy and hard nut he / she will have to stay for one complete year. But if the inmate is sick then he or she can count his/her time of hospitalization as spent in jail.