In Project Management, Time Management comes in first place with the project definition. If you see the definition of the project –
A project is a temporary endeavor undertaken to create a unique product, service, or result. 
“Temporary” is the key word used to show the importance of Time Management in Project Management. Developing & managing schedule is always challenging for a Project Manager. All activities should be completed within stipulated time to come up with unique result or service. This is what PMBOK says –
“Project Time Management includes the processes required to accomplish timely completion of the project”
Let us review the Time Management processes briefly:
Project Time Management consists of 6 major processes. 5 of them are under Planning Process Group & 1 process comes under Monitoring & Control Process Group.
Activity Definition: Identifying & Defining all specific activities that is needed to produce project deliverables (Planning Process Group)
Activity Sequencing: Identifying dependencies between project activities and documenting the sequence in which they need to be executed to achieve the project output in time (Planning Process Group)
Activity Resource Estimating: Resources are men, materials and machines that are required to execute different project activities. Determining what different type of resource required, in what quantity and when each resource will be available to perform the activities is worked out in this process(Planning Process Group)
Activity Duration Estimating: In this process, estimation of required time is done to carry out a defined activity based on the scope & resource availability. Accuracy of this process leads to exact schedule development of whole project & cost estimation of resources (Planning Process Group)
Schedule Development: Coming up with planned start and end dates for every activity & also for the entire project happens in this process. This process uses activity sequences, duration estimation, resource estimation & project schedule constraints to arrive the project schedule. Few well established techniques are already available. Output of this process is approved and used as schedule baseline against which project progress is tracked (Planning Process Group)
Schedule Control: This process explains project status monitoring, finding changes in the schedule(early you find lesser the impact!) and controlling schedule changes to meet planned schedule. Communication is more important in this process (Monitoring & Controlling Process Group)
One can notice high interdependency between time management processes. It is not required that all these processes to be defined separately. In some cases like a project with smaller scope may have all time management processes combined in a single process.
. PMBOK® Guide – 3rd edition