Abstract: le itself is also a “productive misunderstanding” derived from material deduction. From the legitimacy perspective, the right holder of set-off does not have protectable trust on retroactivity, which does not align with the function of set-off, and recognition of bankruptcy set-off cannot support retroactivity either. Therefore, the non-retroactive model established in Article 55 of Interpretation of the General Provisions of Contracts has substantive rationality. The non-retroactive rule is default rule, fitting the “majority default”, and parties can exclude the non-retroactivity specially. On the constitution, the effect of non-retroactivity points to the future when the declaration of set-off arrives, where the set-off requirements must still be met at the time of declaration receipt. Whether allowing set-off with time-barred claims is an issue at the requirement level. The result of non-retroactivity is that the “time when set-off is appropriate” has no special effect at the effectiveness level. In specific effects, non-retroactivity means that interest and penalty fees continue to accrue until the set-off declaration is received, and delay also extends to that point in time. In systemic effects, scenarios such as litigation set-off and enforcement set-off should adhere to non-retroactivity, and non-retroactivity should still take effect from the “time when the set-off declaration is received”. On this basis, time points such as “when the complaint or counterclaim copy is served or when the defense opinion is received” should be recognized as the “time when the set-off declaration is received”.