Reference periods are defined as maximum periods. This means that the employer may set a shorter reference period within these limits.

The maximum length of reference periods for working time is as follows:

• up to 4 weeks – when working continuously,
• up to 1 month – in an equivalent working time system,
• up to 3 months – in an equivalent working time system, the application of which must be justified by special cases,
• up to 4 months – basic working time system, intermittent working time system, equivalent working time system (for work dependent on a season or weather conditions),
• up to 1 month only in the following systems: equivalent working time for work involving the supervision of equipment or involving partial standby duty, and for employees engaged in the guarding of property or the protection of persons, as well as for employees of on-site fire brigades and on-site rescue services; shortened working week and weekend work,
• up to 12 months – in any working time system – if this is justified by objective or technical reasons or reasons relating to work organisation, in accordance with the general rules on the protection of employees’ health and safety.

The working time reference periods adopted by an employer must be laid down in a collective agreement or work regulations or in a notice. The extension of the reference period of working time up to 12 months must be established in a collective agreement or in an agreement with trade union organisations or in an agreement concluded with employee representatives, selected in accordance with the procedure adopted by the employer – if there are no workplace trade union organisations.