While Department of Justice policy counsels against indicting a sitting president, a president can be indicted once out of office. But every crime carries a statute of limitations — a time limit, starting from the day a person commits a crime, within which criminal charges must be brought.
For most federal crimes, the statute of limitations is five years. So if Trump leaves office in January 2021, a five-year statute of limitations would prohibit criminal charges for any conduct occurring before January 2016; if he leaves office in January 2025, then the statute of limitations would bar charges on anything he did before January 2020.
There is, however, an important exception: In some instances, federal law permits "tolling" of the statute of limitations — essentially a pause on the countdown clock. Tolling happens, for example, when a defendant is a fugitive or during the pendency of a request from the United States to a foreign country for evidence.
While the issue has never been decided in court, prosecutors likely would argue that the statute of limitations should be tolled while the President is in office. Prosecutors might make the unusual argument that being president is, for legal purposes, the same as being a fugitive: In both cases, the subject cannot be charged and apprehended, through no fault of the prosecution. Essentially, prosecutors might argue, the President — by virtue of his position — hides from the law, in plain sight.
On the other hand, a defendant likely would argue that the statute of limitations is not tolled if, in effect, the Justice Department prevents itself from charging. The prohibition on indicting a sitting president is not imposed on the Justice Department by the Constitution, statute or judicial decision; it is a prudential limit that the Justice Department has imposed on itself. Such a self-limitation, a defendant would argue, does not toll the statute of limitations.