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Fix Microsoft Project Subtask Duration Not Matching Task Dates

Olivia Miller Oct 9, 2026 872 views

Question details

User needs to fix an issue where the displayed task and summary task durations do not align with the actual start and finish dates.

Fix Microsoft Project Subtask Duration Not Matching Task Dates

Product: Microsoft Project
Device & OS: not provided
Scenario: Calculating task durations and scheduling project timelines.
Observed behavior: Subtask durations and summary task dates are mismatched, often caused by manual date entries, calendar constraints, or custom rounding formulas.

Before you start

Verify whether you are using Project Desktop, Project Online, or Project for the web, as the interface for checking task calendars and constraints may vary slightly across versions.

Solution 1Recommended

Use Auto Schedule and Remove Manual Date Constraints

Entering dates manually forces constraints that prevent dynamic scheduling. Relying on durations and task dependencies resolves most mismatch issues.

In Microsoft Project, manually typing a start or finish date creates a constraint (like 'Start No Earlier Than'). This breaks the dynamic calculation of your project schedule.

Highlight affected tasks

Select the subtasks in your project schedule that are displaying incorrect durations.

Switch to Auto Schedule

Navigate to the Task tab on the ribbon and click 'Auto Schedule' in the Tasks group to enable dynamic calculation.

Remove manual constraints

Double-click a task to open the Task Information dialog. Go to the Advanced tab and change the Constraint Type to 'As Soon As Possible'.

Set durations and dependencies

Instead of typing dates, enter the required working time in the Duration column (e.g., '3 days') and define task relationships using the Predecessors column.

Use Auto Schedule and Remove Manual Date Constraints

Best Practice: Always let Microsoft Project calculate the dates based on your task durations and dependencies to maintain an accurate and flexible schedule.

Solution 2

Check Project and Task Calendars

Conflicting working times between the base project calendar and individual task calendars can cause the duration calculation to misalign with the calendar dates.

Solution 3

Verify Summary Task Calculations and Custom Fields

Summary tasks are automatically calculated based on subtasks. Additionally, custom duration fields might be using rounding formulas that alter the displayed numbers.

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1. Download and Install: Get WPS Office for free from the official website and install it on your device.
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Frequently Asked Questions

Why does my summary task duration show more days than the sum of its subtasks?

Summary task duration represents the total working time from the earliest start date of the first subtask to the latest finish date of the last subtask. It includes gaps between tasks and accounts for working days, which is why it often differs from the simple mathematical sum of the subtasks.

How do I stop Microsoft Project from changing my dates?

Microsoft Project is designed to schedule dynamically. When you manually enter dates, it creates restrictive constraints. To allow proper dynamic scheduling, change your tasks to 'Auto Schedule' and use task dependencies (the Predecessors column) to drive the schedule instead of typing fixed dates.

Can I use a rounding formula for the duration field?

There is no standard rounding formula available for the default Duration field. If you require rounded durations, you must create a custom duration field (such as Duration1) and apply your custom mathematical formula to that specific field.

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