Others

Fix Microsoft Project Changing Task Durations When Base Calendar Changes

Maira Mehtab Sep 24, 2026 869 views

Question details

The user needs task durations to remain fixed and only task dates to adjust when switching between different holiday calendars (e.g., Canadian and United States) in Microsoft Project.

Product: Microsoft Project 2019
Device & OS: not provided
Scenario: Switching project base calendars to account for regional holidays while maintaining accurate task durations.
Observed behavior: Switching the base calendar changes the actual task durations instead of shifting the task start or finish dates.

Before you start

Before modifying your task types or base calendar, take a screenshot of your current Gantt Chart or save a project baseline so you can accurately compare the schedule before and after the change.

Solution 1Recommended

Change Task Types to Fixed Duration

Set your tasks to 'Fixed Duration' to ensure that changing working days via the base calendar only recalculates the task dates, not the duration itself.

Microsoft Project calculates schedules based on the relationship between Work, Duration, and Units. When the calendar changes available working hours, Project may adjust duration to preserve the assigned work effort. Setting the task to Fixed Duration overrides this behavior.

Select the target tasks

Open your project and select the tasks you want to modify in the Gantt Chart view.

Open Task Information

Right-click the selected tasks and choose 'Information' to open the Task Information dialog box.

Set Fixed Duration

Navigate to the 'Advanced' tab, locate the 'Task type' dropdown menu, and change it to 'Fixed Duration'.

Apply the new calendar

Switch your project base calendar to the new regional holiday calendar and verify that the finish dates move while durations stay exactly the same.

Tip: If you want all future tasks to default to this behavior, go to File > Options > Schedule and set the 'Default task type' to 'Fixed Duration'.

Solution 2

Review Resource Assignments and Effort-Driven Settings

Check if your tasks are effort-driven, as resource assignments can influence how Microsoft Project recalculates durations when calendar working hours change.

Free Microsoft Office alternative

Manage Project Timelines Easily with WPS Office

While Microsoft Project offers advanced scheduling capabilities, it can be overly complex for standard project tracking and can lead to unexpected duration changes. For a simpler approach, WPS Spreadsheet is an excellent, free alternative for building Gantt charts and tracking project milestones without the complicated calculation engine.

1. Download and Install WPS Office: Get the free WPS Office suite from the official website and install it on your device.
2. Open a Project Template: Launch WPS Spreadsheet, go to the Template library, and search for 'Gantt Chart' or 'Project Tracker'.
3. Customize Your Timeline: Input your fixed task durations, start dates, and milestones to create a reliable and easily adjustable project schedule.

Completely free and lightweight office suiteFully compatible with Microsoft Excel (XLSX) project tracking templatesIntuitive interface that avoids unexpected auto-calculations in task durationsRich built-in template library for Gantt charts and team timelines

Frequently Asked Questions

Why does Microsoft Project change duration instead of dates when calendars change?

By default, Microsoft Project attempts to keep the total amount of 'Work' constant. If a calendar change removes working days (like adding a holiday), Project might decrease or increase the duration to maintain the same total hours of effort, unless you specifically define the task as 'Fixed Duration'.

What is a Fixed Duration task in Microsoft Project?

A Fixed Duration task is a setting where the amount of time assigned to complete the task remains constant. If you add or remove resources, or change the base calendar's working days, Project will adjust the workload or the dates, but the duration value itself will not change.

How do I change the base calendar for an entire project?

To change the project's base calendar, go to the 'Project' tab on the ribbon, click 'Project Information', and select your preferred calendar from the 'Calendar' dropdown list.

Does changing the project calendar affect existing resource calendars?

Not necessarily. Individual resource calendars might be based on the old base calendar. You should verify resource calendars via the 'Change Working Time' dialog to ensure they inherit from the newly selected base calendar.

Office Quick Solutions