Annualising working hours (AH) is a mean to face the seasonal nature of the demand. AH provides much flexibility to the production system, so the company is allowed to plan the staff working time more efficiently. However, the introduction of AH entails new optimisation problems to solve, as is the working time planning problem. These problems should be solved in an efficient way so AH could be a real and operative flexibility tool. Mixed and integer linear programming has been shown as a very effective approach in the resolution of some real and artificial generated AH problems.