Time is typically treated as a background parameter within which physical processes occur. This work instead addresses a prior question: what structural conditions must be satisfied for time to become meaningful at all? We propose that the opening of time is not a dynamical process but a two-stage structural condition. First, time opens formally through the admissibility of coherent phase histories, independent of clocks, motion, or physical media. Second, time opens physically when a medium permits persistent registrability, allowing admissible histories to survive as records. These conditions are governed by existing physical formalisms, which are not re-derived here but re-situated as prerequisites for registrability rather than as laws operating within time. Matter, degrees of freedom, and thermodynamic behavior arise only downstream of this opening, as consequences of recoverable structure. This framework separates time, registrability, and dynamics; clarifies the role of plasma transparency and recombination; and provides a falsifiable ordering principle for early-universe structure without invoking formation narratives or speculative mechanisms.