Equitable tolling is a principle of law stating that a statute of limitations shall not bar a claim in cases where the plaintiff, despite use of due diligence, could not or did not discover the injury until after the expiration of the limitations period. When the United States is a defendant, equitable tolling cannot be applied against the United States since the Spending Clause has been interpreted by the Supreme Court to only vest Congress with the authority to waive sovereign immunity, and statutes of limitation are interpreted as a condition on the waiver of sovereign immunity that limit the jurisdiction of a court to hear cases against the United States.