In the case of De Agbayani v. Philippine National Bank, 38 SCRA 429 , the Court discussed the effect to be given to a legislative or executive act subsequently declared invalid:
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. . . It does not admit of doubt that prior to the declaration of nullity such challenged legislative or executive act must have been in force and had to be complied with. This is so as until after the judiciary, in an appropriate case, declares its invalidity, it is entitled to obedience and respect. Parties may have acted under it and may have changed their positions. What could be more fitting than that in a subsequent litigation regard be had to what has been done while such legislative or executive act was in operation and presumed to be valid in all respects. It is now accepted as a doctrine that prior to its being nullified, its existence as a fact must be reckoned with. This is merely to reflect awareness that precisely because the judiciary is the government organ which has the final say on whether or not a legislative or executive measure is valid, a period of time may have elapsed before it can exercise the power of judicial review that may lead to a declaration of nullity. It would be to deprive the law of its quality of fairness and justice then, if there be no recognition of what had transpired prior to such adjudication.
In the language of an American Supreme Court decision: "The actual existence of a statute, prior to such a determination of [unconstitutionality], is an operative fact and may have consequences which cannot justly be ignored. The past cannot always be erased by a new judicial declaration. The effect of the subsequent ruling as to invalidity may have to be considered in various aspects, — with respect to particular relations, individual and corporate, and particular conduct, private and official." (Chicot County Drainage Dist. v. Baxter States Bank, 308 US 371, 374 ). This language has been quoted with approval in a resolution in Araneta v. Hill (93 Phil. 1002 ) and the decision in Manila Motor Co., Inc. v. Flores (99 Phil. 738 ). An even more recent instance is the opinion of Justice Zaldivar speaking for the Court in Fernandez v. Cuerva and Co. (21 SCRA 1095 . (At pp. 434-435)
The "operative fact" doctrine realizes that in declaring a law or rule null and void, undue harshness and resulting unfairness must be avoided. It is now almost the end of 1991. To require various companies to reach back to 1975 now and nullify acts done in good faith is unduly harsh. 1984 is a fairer reckoning period under the facts of this case.