What is the Difference Between Calendar Days and Working Days in Primavera P6

In construction project planning and scheduling, clarity in timelines is essential—not only for execution but also for communication, reporting, and contractual compliance. One of the most common sources of confusion lies in the interpretation of calendar days versus working days. Although both are used to measure durations, they differ significantly in their impact on project timelines, resource planning, and schedule logic.

Whether you’re managing contracts, setting deadlines, or building schedules in software like Primavera P6, understanding this distinction is critical. In this article, we’ll break down the differences between calendar days and working days, explore how they influence scheduling, and how to show Calendar Days instead of Working Days in P6 WBS Summary.

What are the Calendar days and Working Days?

Calendar Days:

Calendar days refer to all days on the calendar, including Weekdays (Monday to Friday), weekends (Saturday and Sunday), and public holidays. Everyday counts, regardless of whether work is performed or not.

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Example:
If a task is scheduled for 10 calendar days, and it starts on June 1, it is expected to finish on June 10, including weekends.

Working Days:

Working days refer to days when work is planned to be performed, based on the project calendar. This calendar may exclude weekends (e.g., Friday/Saturday or Saturday/Sunday) or exclude national/public holidays.

Example:

If a task is scheduled for 10 working days and the workweek is Sunday to Thursday, starting on June 2 (Sunday), the task would finish on June 13 (Thursday).

If you want to learn how to add holidays to the calendar in a smart way on Primavera P6, you can follow the steps in this article: How to Add Holidays to P6 Calendar?, Or if you already have a calendar and you want to copy it to your new project, you can follow the steps in this article: How to Copy a Project Calendar from one Project to another.

Case Study on How to Show Calendar Days instead of Working Days

As can be seen in the screenshot below, the project is planned to start on 4-April-2021 and finish on 30-March-2024.

If we calculated the days between those days on Excel, we would find that it is 1092 days (Calendar days) and not 890 days (Working days). This could lead to contractual problems between the contractor and the consultant if the contract states the project durations in days.

Note: the +1 is to include all days (finish date should be included)

So, to show the actual calendar days instead of working days to match the dates in the start and finish columns, we need to go to the project settings. And press the “…” next to calendar and choose a 7 working days calendar.

Press on the (7 Days) and then choose “Select” which is the plus sign.

Now, when we open the project, we will find that the duration column is 1092 days.

As a planner, now you know that if the contract states the duration in calendar days and they give you the commencement day, you can calculate the project duration on Primavera P6 and show the calendar duration in the schedule to avoid any misleading or contractual complications.

You can check this video for further elaboration: How to Show Calendar Days instead of Working Days in P6 WBS Summary

Conclusion

Understanding the distinction between calendar days and working days is essential for effective planning and scheduling in construction projects. While calendar days include every day on the calendar (weekends and holidays included), working days are limited to days when actual site work is planned, based on the project’s working calendar.

For project planners, especially those using Primavera P6, recognizing this difference is critical to avoid miscommunication, incorrect schedule durations, and unrealistic expectations. Primavera P6 allows planners to assign specific calendars to activities, ensuring accurate reflection of working hours, non-working days, and holidays. Therefore, to build a reliable and realistic schedule, planners must always align the activity durations and calendar types with the actual conditions of the project and contractual requirements.

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