Each of the four petitioners below—two appellants, two respondents here—resigned from the police department under honorable circumstances, with spotless records, for reasons having nothing whatever to do with their performance as officers. Following periods of separation from the department of about 10 months—9 in one case—each applied for reinstatement. In each instance, formal reinstate*396ment came about with seniority running from the date thereof, with the same rank, grade, and salary as before; the later dating of seniority without continuity from the original date of induction was to plague petitioners later on when the city fell into financial difficulties and policemen were laid off, with seniority being the key factor in the order of their going. The problem before us derives from the seniority date: had each petitioner’s seniority run from the original date, none of them would have been discharged; by reason of the later date, each had his seniority truncated to the point where he was junior to others privileged to remain. Each has been aggrieved by the fiscally compelled discharge, and each has, therefore, commenced a proceeding pursuant to CPLR article 78 to compel the Police Commissioner, his department, the Department of Personnel, and the City Civil Service Commission to accord him continuity of service so as to give him seniority running from the date of original induction as patrolman. The statute and rules involved in reinstatement of resigned employees are correctly set forth in the dissent and require no repetition here. They must be read together as one. It is obvious that an employee reinstated by the commissioner’s favor—a word used advisedly because reinstatement lies within his discretion—is entitled, if there is compliance with the expressed limitation of time in rule 6.2.1 of the City Civil Service Commission, to the benefit of section 80 of the Civil Service Law in respect of continuity of service as though there had never been a break in employment.
The facts, in addition to those stated above, which bear out the conclusion we reach of entitlement of petitioners to the relief sought, are few and simple. As has been said before, each applied for reinstatement in two to three months less than a year’s time. It is stated in the dissent, and it is so, that each of the four applications was viewed with favor at the threshold and appropriate departmental proceedings initiated to formalize return of petitioners to the ranks; these consisted of physical examinations, as well as investigations of the activities of the petitioners during their months of separation from the department. The proceedings in respect to petition Atkatsh were terminated favorably two days before the anniversary of his resignation; administrative departmental procedure postponed actual reinstatement until the lapse of another month thereafter. Petitioner McCabe passed the physical examination a month after his application but the depart*397ment, which perhaps possesses the greatest expertise anywhere in this field, took some six weeks to process his fingerprints; his recall to duty was postponed until some time after a year had elapsed after resignation. Petitioner Franchina’s • return to duty was postponed until two months after the year was up; the department was investigating his home activities for some weeks in Nassau County before discovering that his residence was in Suffolk! Petitioner Moore’s anniversary date came and went without formal reinstatement taking place until more than three months thereafter, although he pointed out to the department that he had just been through as rigorous an investigation by the department itself in connection with his application for a pistol permit. The two respondent petitioners were sustained below as to the relief sought; the two appellant petitioners were denied relief. All four are entitled to the relief sought.
It is to be emphasized that each of these men went through his physical examination with little delay and was then marked qualified for reinstatement. The commissioner did not have to do this; the petitioners were never rejected at this point—indeed, never rejected at all. A litany of cases is recited in the dissent, each standing for the proposition that the commissioner may reject without a reason. These are the same cases set forth in the department’s briefs, and cited by the two Justices below who decided against two of the petitioners. With this rule petitioners have no quarrel, but it is not actually here involved. What is involved is the arbitrary and unreasonable conduct of the department in enmeshing petitioners in a Laocoon-like entanglement of police department green tape, which unreasonably delayed matters to the extent that each petitioner was thereby deprived of his right to section 80 continuity. Each investigation could easily have been completed within the time afforded, but it was not completed timely. Whether this was for lack of due diligence or insensitive disregard of petitioners’ rights matters not. One of the Justices below observed in denying relief to a petitioner, that "petitioner cannot complain about the time it took to perform this voluntary act since he could not compel the act to be performed at all.” This is just not so. The act was actually performed, but in such a manner and with such unreasonable delay in each case as to deprive each "beneficiary” of its beneficial effect.
A public official undertaking to perform an act, even an act *398which is completely discretionary, must do so reasonably and in complete good faith without such delay as would frustrate its ultimate objective. As the dissent concedes, "the discretion of the Police Commissioner * * * is not absolute in the sense that no limitation may be placed upon it.” "Arbitrary action cannot elude judicial reach by the plea that it was no more than the use of proper administrative discretion. The moment it is shown to be arbitrary, it ceases to be discretionary”. (Matter of Nathanson v Adams, 207 Misc 572, 576-577.) (See, also, Forkosch, Administrative Law, Scope of Judicial Review § 338[a].) We do not hold that the exercise of discretion evaporated the instant the commissioner made the threshold determination that each petitioner was acceptable for return to the department. He could have, as he did not, changed his mind at any time before the oath was administered. But, having embarked upon the course of examination and investigation after initial acceptance, it was the commissioner’s duty to proceed with the despatch required in the circumstances. If he proceeded without regard to the approaching deadline, he was negligent; if with knowledge of the deadline and its possible impact upon the application he was dilatory, then he was unreasonable. (See Calzaretta v Mulrain, 131 NYS 2d 76, 80; Matter of Ormsby v Bell, 218 NY 212, 216.) "The power to review agency action necessarily implies the power to reverse, modify, or set aside aspects of such action which violate an applicable legal rule or principle of justice. Thus, unduly prolonged proceedings * * * [are] subject to judicial invalidation.” (72 Yale LJ 574, 588-589, The Right to Relief from Unduly Protracted Proceedings.) Add to the discretion of the Police Commissioner that of the Civil Service Commission. In the circumstances found, the latter could not possibly have disagreed with the Police Commissioner’s finding that petitioners were acceptable for service. Its part in the events was actually no more than performance of a ministerial act in consenting to reinstatement. To whatever unknown extent there may have been contributory delay by civil service, its delay would have also been unreasonable. Nor do we say that any delay at all would be unreasonable for each case of alleged unreasonable exercise of discretion must be measured ad hoc against its own peculiar set of circumstances. What we do say is that, in these four cases, the delay in processing the applications for reinstatement was unreasonable.
It is unrealistic to. contend that, if petitioners were ag*399grieved at all, it was at the moment of delayed return to service with consequent losss of seniority. How aggrieved then? They had been restored in every respect except seniority date to the position theretofore occupied. Says our dissenter that "petitioners knew, or are deemed to have known, the effective dates of their reinstatement and that such dates were without the one-year period necessary to preserve continuity of service.” This assumption is correct only in part; they knew the dates. But how could they possibly have known of the effect of these dates until the dread day that their livelihood was snatched from them? Not even the highest and most knowledgeable of city officials knew in 1972 and 1973 that, a few short years later, layoffs of experienced police officers would be required for economic reasons. The day of aggrievement in each case was the date of discharge, and all four cases are outside the bar of CPLR 217.
We hold all four petitioners entitled to reinstatement, but not without limitation as to back pay from discharge to restoration of duty. Each must suffer diminution of his award to the extent of mitigation by the amount of compensation earned in any other employment or occupation during that period. (Matter of Ector v Salzmann, 54 AD2d 1017.) There must also be considered the period between the dates of the orders here reviewed (two in the fall of 1975; two in the spring of 1976) and the calendaring of these appeals for the April 1977 Term. Should responsibility for this extraordinarily long delay be attributable to petitioners, it should be taken into account also by diminution of each award. The cases will therefore be remanded to Special Term, to hold a hearing to evaluate these factors and their impact on the amount of back pay, and then to enter a final judgment inclusive of that amount of back pay which may be due and owing to petitioners in each of the four cases.
Accordingly, the judgments of Supreme Court, New York County (Helman, J., and Postel, J.), entered respectively on March 4 and June 1, 1976, relating to petitioners- appellants Moore and Franchina, should be reversed, on the law and each such respective petition for reinstatement granted, and the matters remanded to Special Term, New York County, for a hearing to fix the amount of back pay owing to each petitioner-appellant in accordance with this opinion, for inclusion in the respective judgments to be entered in each proceeding; and the judgments of the same court (Gellinoff, J.), *400entered October 1 and 3, 1975, granting the petitions of petitioners-respondents Atkatsh and McCabe for reinstatement, should be modified, on the law, to the extent of remanding the matters to Special Term, New York County, for a hearing, consolidated with the hearing above ordered, to fix the amount of back pay owing to each petitioner-respondent in accordance with this opinion, for inclusion in the judgment to be entered in favor of each, and otherwise those judgments should be affirmed; all the foregoing to be without costs and without disbursements in each of the four cases.