Among the triple constraints of time, cost and scope, time is least flexible in scrum. In scrum everything is time boxed. A sprint planning meeting must get over within eight hours, if the sprint duration is thirty days. If the sprint duration is lesser than thirty days, then the sprint planning meeting duration can reduce, still must get over within the agreed upon time, else it is considered as failure. The sprint must get over within the agreed upon sprint duration. Not a single days extension is allowed. The sprint review meeting must get over within the agreed upon time, so is the case with the retrospective meetings. So the knowledge area of time management has a very high significance in agile. Word of caution..the concept of critical path is not advocated within a sprint as this hampers the concept of self organising teams. The scrum master is never allowed to provide an activity on node, or activity on arrow diagram or a gantt chart of the activities to be performed within a sprint to the team. The team will decide how they are going to do it. During the sprint planning meeting the sequence of development at the feature level is determined and it stops there. Task level sequencing by the scrum master for the team within a sprint is prohibited, as it hampers the concept of self organising teams.
For a complete description of scrum download the scrum guide from http://www.scrum.org