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How to Fix Microsoft Project Calculating the Wrong Finish Date

Steve K Oct 9, 2026 870 views

Question details

The user needs to resolve an issue where Microsoft Project calculates task finish dates incorrectly, adding extra time to the scheduled task duration.

How to Fix Microsoft Project Calculating the Wrong Finish Date

Product: Microsoft Project
Device & OS: not provided
Scenario: Planning a project schedule and assigning duration times to specific tasks.
Observed behavior: A three-day task starting on January 29 finishes on February 1 instead of January 31, and this calculation error persists throughout the entire project schedule.

Before you start

Ensure you have administrative access to your project file to modify project calendars. It is also highly recommended to display the time alongside dates in the Start and Finish columns to see exactly when tasks are scheduled to begin and end.

Solution 1Recommended

Verify Project and Resource Calendars

Check the working days, hours, and exceptions in your project and resource calendars, as these dictate how task durations are calculated.

Microsoft Project calculates finish dates based on the assigned working times. If there is a mismatch between the default project calendar and the resources' base calendars, or if standard working hours have been modified, tasks may span across more days than expected.

Check Project Working Time

Navigate to the 'Project' tab on the ribbon and click on 'Change Working Time'. Verify that standard working days and hours are configured correctly for the project calendar.

Verify Task Calendars

Double-click the affected task to open the Task Information dialog. Go to the 'Advanced' tab and check if a specific 'Calendar' is overriding the project default.

Check Resource Calendars

Switch to the 'Resource Sheet' view, double-click a resource assigned to the task, and ensure their Base Calendar matches the project calendar without unintended non-working days.

Check Default Options: Always ensure the 'Hours per day' setting in File > Options > Schedule matches the actual working hours defined in your working time calendar.

Solution 2

Adjust Task Type and Scheduling Mode

Task types and scheduling modes can alter how finish dates are calculated when durations or resources are changed.

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Frequently Asked Questions

Why does a 3-day task span over a weekend in Microsoft Project?

If your project calendar defines weekends as non-working days, Microsoft Project will automatically skip them. A 3-day task starting on a Friday will thus finish on the following Tuesday.

How do I change the default hours per day in Microsoft Project?

Go to File > Options > Schedule. Under the 'Calendar options for this project' section, update the 'Hours per day' value to match your organization's standard working hours.

Can assigned resources change the finish date of a task?

Yes. If a task is effort-driven or if the assigned resource has a different base calendar (such as vacation days), Microsoft Project will extend the task's finish date to accommodate their availability.

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