The rule that pension rights of an employee vest at the time of hire is well established. (Miller v. State of California (1977) 18 Cal. 3d 808, 817, 135 Cal. Rptr. 386, 557 P.2d 970; Kern v. City of Long Beach (1947) 29 Cal. 2d 848, 852-853, 179 P.2d 799.) But the rule is qualified. 'A public pension system is subject to the implied qualification that the governing body may make reasonable modifications and changes before the pension becomes payable and that until that time the employee does not have a right to any fixed or definite benefits but only to a substantial or reasonable pension.'" (Miller, supra, at p. 816; International Assn. of Firefighters v. City of San Diego (1983) 34 Cal. 3d 292, 300-301, 193 Cal. Rptr. 871, 667 P.2d 675; Kern, 29 Cal. 2d at pp. 853-855.) Consistent with these principles, the Supreme Court has determined that "since the industrial injury is the basis for any compensation award, the law in force at the time of injury is to be taken as the measure of the injured person's right of recovery." (Aetna Cas. & Surety Co. v. Ind. Acc. Com. (1947) 30 Cal. 2d 388, 392, 182 P.2d 159; see also State Comp. Ins. Fund v. Workers' Comp. Appeals Bd. (1977) 71 Cal. App. 3d 133, 136, 139 Cal. Rptr. 410 [same].) Thus, the vesting of retirement benefits must be distinguished from the maturing of those benefits, which occurs only after the condition(s) precedent to the payment of the benefits has taken place. (See Kern v. City of Long Beach, supra, 29 Cal. 2d at p. 851 [pension benefits vest upon happening of contingency upon which the pension becomes payable]; see also Terry v. City of Berkeley (1953) 41 Cal. 2d 698, 702-703, 263 P.2d 833, and cases cited.) This principle is well-illustrated in Miller v. State of California (1977), supra, 18 Cal. 3d 808. Our Supreme Court held that no contract clause violation occurred when the Legislature reduced the mandatory age of retirement thereby defeating Miller's expectation of a maximum pension. The court reasoned: "Although his right to a pension based on this system was vested, plaintiff was not assured of receiving maximum pension benefits. His right to receive such benefits was subject to conditions and contingencies; specifically, that he remain in state employment until age 70. Plaintiff failed to satisfy that condition since he was lawfully placed on retirement at age 67. Thus, his right to a maximum pension based on retirement at age 70 never matured." (Id. at p. 817.)