A task’s duration represents the amount of time you expect it will take to complete the task. Projects can work with task durations that range from minutes to months. Depending on the scope of your plan, you’ll probably want to work with task durations on the scale of hours, days, and weeks. Giving your task duration values is one of the benefits of using a scheduling tool like Project over a simple checklist or to-do approach to organizing work.
Let's learn a form of duration which is very important for project management.

Elapsed duration: The uninterrupted span of time that it will take to finish a task, based on a 24-hour day and a 7-day week. Elapsed duration is not limited by project, resource, or task calendars; it is continuous.
  • If needed, you can schedule tasks to occur during nonworking as well as working time. To do this, enter an elapsed duration to a task. You enter elapsed duration by preceding the duration abbreviation with an “e”. For example, type “1ed” to indicate one full 24-hour day.
  • You might use an elapsed duration for a task that goes on around the clock rather than just during normal working hours.
  • For instance, a construction project might have the tasks “Pour foundation concrete” and “Remove foundation forms.” If so, you might also want a task called “Wait for the concrete to cure” because you don’t want to remove the forms until the concrete has cured. The task “Wait for the concrete to cure” should have an elapsed duration because the concrete will cure over a contiguous range of days, whether they are working or nonworking days. If the concrete takes 48 hours to cure, you can enter the duration for that task as “2ed,” schedule
    the task to start on Friday at 9 A.M., and expect it to be complete by Sunday at 9 A.M.
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Kailash Upadhyay