Article II, section 11 of the Colorado provides that,
No ex post facto law, nor law impairing the obligation of contracts, or retrospective in its operation, or making any irrevocable grant of special privileges, franchises or immunities, shall be passed by the general assembly.
Bans on ex post facto laws have long been interpreted as applying only to criminal statutes (see, e.g., Calder v. Bull (1798)), but the Colorado Supreme Court had held that bans on retrospective laws also extend to lawsuits dealing with civil liability; and while deciding what counts as a "retrospective" law can be complicated, the court had held that revival of claims on which the statute of limitations had expired is indeed impermissibly "retrospective." In yesterday's Aurora Public Schools v. A.S., the court unanimously held (in an opinion by Justice Monica Márquez) that the Colorado Legislature's revival of time-barred sexual assault claims was thus unconstitutional. And it added,
