A distinction must be drawn between companies which, at the time of entry into force of the Law of 23 December 2016 concerning the organisation of working time, that is to say 1 January 2017:
- (a) are covered by a collective work agreement, a subordinate agreement or an agreement reflecting interprofessional social dialogue which do not contain any provision regarding a reference period or confine themselves to referring to the general law with regard to the organisation of working time;
- (b) are covered by a collective work agreement, a subordinate agreement or an agreement reflecting interprofessional social dialogue which contain provisions regarding a reference period;
- (c) are not covered by a collective work agreement, a subordinate agreement or an agreement reflecting interprofessional social dialogue.
For the companies referred to in paragraph a), the legal reference period applicable until the expiry of the collective agreement or another agreement may not exceed one month, given that the provisions thereof refer to the general law which was applicable at the time of conclusion of the collective or other agreement.
For the companies referred to in paragraph b), the provisions on reference periods provided for by the collective agreement or other agreement will remain applicable until the expiry of that collective or other agreement.
The companies referred to in paragraph c) may decide to apply a reference period which may be of up to 4 months.
Any collective or other agreement which is to be put in place after the entry into force of the Law of 23 December 2016 concerning the organisation of working time, that is to say 1 January 2017, may lay down provisions concerning the reference period, the duration of which may be reduced to less than the legal reference period of 4 months or indeed be extended up to a maximum of 12 months.
Where the collective or other agreement which has been adopted after 1 January 2017 does not provide for provisions concerning the reference period or merely refers to the general law regarding the organisation of working time, the reference period may not exceed 4 months.
The reference period may be expressed in weeks or in calendar months.
Having regard to the duration of the chosen reference period, companies must comply with other legal requirements (for example: additional days of leave, shorter maximum working times).