All Planning Engineers, and most of project managers can create a Gantt Chart (Bar Chart) using different scheduling tools such as Primavera and Microsoft Project. However, only few of them really know how the scheduling software is calculating the network diagram in order to estimate the project duration, project floats, and critical path.
The duration task estimation is a challenge target to the project team. It does not has neither fixed rules nor clear assumptions to follow, it is related to task resources allocation, and could vary from person to another and from project to another. (Wysocki 2012)