It must be added that a new legal rule applies from the entry into force of the act introducing it, and that, while it does not apply to legal situations that arose and became definitive prior to that entry into force, it does apply immediately to the future effects of a situation which arose under the old law, and to new legal situations. The position is otherwise, subject to the principle of the non-retroactivity of legal acts, only if the new rule is accompanied by special provisions which specifically lay down its conditions of temporal application (see, to that effect, judgment of 26 March 2015, Commission v Moravia Gas Storage, C‑596/13 P , EU:C:2015:203 , paragraph 32 and the case-law cited).

It should be recalled that, according to settled case-law, procedural rules are generally taken to apply from the date on which they enter into force (judgment of 11 December 2012, Commission v Spain, C‑610/10 , EU:C:2012:781 , paragraph 45 ), unlike substantive rules, which are usually interpreted as applying to situations existing before their entry into force only in so far as it follows clearly from their terms, their objectives or their general scheme that such an effect must be given to them (see, to that effect, judgments of 12 November 1981, Meridionale Industria Salumi and Others, 212/80 to 217/80 , EU:C:1981:270 , paragraph 9 , and of 23 February 2006, Molenbergnatie, C‑201/04 , EU:C:2006:136 , paragraph 31 ).

Council Directive 97/81/EC of 15 December 1997 concerning the Framework Agreement on part-time work concluded by UNICE, CEEP and the ETUC, as amended by Council Directive 98/23/EC of 7 April 1998, must be interpreted as meaning that, in a case such as that at issue in the main proceedings, periods of service prior to the deadline for transposing Directive 97/81, as amended by Directive 98/23, must be taken into account for the purpose of calculating the retirement pension entitlement

In the present case, it must be stated that the United Kingdom Government at no time requested that the Court limit the temporal effects of its judgment of 1 March 2012, O’Brien (C‑393/10, EU:C:2012:110). A restriction of that kind may be permitted only in the actual judgment which gives the ruling on the interpretation requested (see, to that effect, judgment of 17 May 1990, Barber, C‑262/88, EU:C:1990:209, paragraph 41).

Moreover, with regard to the argument of the United Kingdom Government that the calculation of the period of service required to qualify for a retirement pension should be distinguished from the rights to a pension, it must be noted that it cannot be concluded from the fact that a right to a pension is definitively acquired at the end of a corresponding period of service that the legal situation of the worker must be considered definitive. It should be noted in this respect that it is only subsequently and by taking into account relevant periods of service that the worker can effectively avail himself of that right with a view to payment of his retirement pension.

Consequently, in a situation such as that in the main proceedings, in which the accrual of pension entitlement extends over periods both prior to and after the deadline for transposition of Directive 97/81, it should be considered that the calculation of those rights is governed by the provisions of that directive, including with regard to the periods of service prior to its entry into force.

(see paras 34-36, 38, operative part)