Discuss the difference between constraints and deadlines. Why is it important to know the difference?

Duration estimation is “The quantitative assessments of the likely number of time periods that are required to complete an activity”.

Duration estimation uses information from the scope of work, required resource types or skill levels, estimated resources quantities, and resource calendars. It is progressively elaborated and considers the quality and availability of the input data. This process requires an estimation of the amount of work effort required to complete the activity and the amount of available resources estimated to complete it. The number of resources that are expected to be available to accomplish an activity and the skill proficiency of those resources can determine the activity’s duration.

Is estimating the duration of project activities difficult? Why or why not?
Discuss the difference between constraints and deadlines. Why is it important to know the difference?