In short, jail time calculation runs from when person is in jail. Yet, there several things that could ultimately reduce the amount of time that he serves. For example, a person is usually given credit for all time already served on that charge. Further, depending on the particular jail and what happens while the person is incarcerated, sometimes a person can have the time actually reduced through "good time" calculations or other programs, such as work release programs.
Your boyfriend knows the exact date--the jail will inform him of his release date. If you visit him or he calls you, tell him to ask the jail staff.