The scrum framework is a simple framework for delivering projects. Although it is targeted towards complex projects, its principles can be used to ensure that any project or task is delivered on time. In scrum, delivering a project "on-time" means that those involved commit to and deliver a set of features by the end of a defined period called a sprint. When the product owner is happy that enough "sprints-worth" of features have been delivered he or she can make a decision to release the product. Delivering on commitments regularly gives confidence that a release is on track.
The challenge in delivering each sprint "on-time" comes from the fact that each team and individual comes with their own characteristics and each sprint comes with its own challenges. Therefore, there is a high degree of unpredictability involved in meeting sprint commitments.