Usually, the actual duration of construction projects significantly exceeds the scheduled duration. Reasons for this phenomenon are presented. Firstly, numerous stochastic factors impact on the works. The second reason is insufficiently reliability of traditional scheduling methods. Finally, the third reason is quasi-activities that were not included in the schedule. This paper discloses the essence quasiactivities, their impact on the completion times. The approach is identified additional dummy arcs, causing implicit activities. The general applicability of the method is demonstrated. A comparison was drawn between the proposed method and traditional techniques. The mean duration of the simple chain of activities is underestimated by 15-20%.It is confirmed that the traditional method of calculating the time to complete a project is almost always shorter. Implementation of this method will allow for the determination of a more precise duration for the performance of complex works at the planning stage. The suggested methodology can be recommended for use by construction project managers.