I think that the planning process should be done in a way where vacation and certain absences from employees will not derail the timeline. What I am saying is that there should be some cushion for employees to be absent, even unplanned absences. For example, for an employee with children it should be taken into consideration that kids get sick. Let say about 3 times a year. So for that employee maybe it should be calculated that in a year long project, he/she might have an unplanned absence 3 times. He/she might be absent less times, which is great, or more times. Of course, theres certain scenarios that can not be planned for such as a very severe accident. I am not sure if doing schedules like this would be an HR problem. But, I just think tailoring timelines this way would result in more accurate timelines.