The answer depends on what triggered that 100% designation, and it is an important distinction.
If the 24 months at 100% is tied to a probation or parole violation, it means exactly what it sounds like. He serves every day of that sentence with no good time, no early release, no parole consideration. When the court sees that a previous grant of leniency was not honored, the response is typically to close that door. In that case, yes, January 4th two years out would be his release date, and there is nothing to reduce it further.
If, on the other hand, this is the original Judgment and Commitment order signed by the sentencing judge on a new case, the picture looks different. Federal inmates and many state inmates are automatically entitled to a good time credit when they begin their sentence. In the federal system, that figure is 15 percent, applied automatically from day one. He does not earn it over time. It is his from the start and can only be taken away if he picks up disciplinary infractions. On a 24-month sentence, 15 percent works out to roughly 50 days off his release date, which would move that January 4th date up by about seven weeks.
The clearest way to know which situation applies is to look at the paperwork carefully. If it references a violation of supervision, the 100% is likely punitive and absolute. If it is a straight sentencing order on a new charge, the good time credit almost certainly applies. His attorney or the facility's case manager can confirm which category he falls into.