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MS Project 2010 tries to be smart and tries to figure out correct dates and times.

Unsurprisingly, it fails miserably once scheduling becomes just a little unusual. So now I have the following problem:

  1. When I set the end date right, MS Project changes the time it takes to something wrong.
  2. When I set the time it takes right, MS Project changes the end date to something wrong.

Every time I set one of them to the actual correct value the other is set to a wrong value.

What can I do? What I would like best would be to just turn off all automatic guessing.

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You probably want to set your task to "Fixed Duration" mode (double click on the task -> Advanced -> Task Type). You can also set "Fixed Duration" as the default mode for tasks in Options menu.

Here is why: MS-Project will always maintain the following equation consistent:

Duration = Work / Resources

  • Duration are the working days between the start and end date of your task
  • Work are the estimated number of hours it takes to complete the task and
  • Resources is 100% if you assign one guy.

So whatever you touch, MS-Project will adjust the other values to make the equation consistent. Selecting "Fixed Duration" you tell MS-Project to modify the resource assignation value if you change the duration of the task. That's probably what you want.

Cheers, Frank

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Frank's answer is correct but left out one important detail. MS Project versions prior to 2010 are not designed to support manual entry of start and end dates; this is the single biggest mistake new users of the app make and causes the most frustration. This is a powerful tool that is not Excel on steroids so don't try to use it that way.

The proper way to use MS Project, in keeping with its design, is to establish a start date for the project in Project Information then create a series of linked, dependent tasks with defined work (hours) or duration (elapsed calendar time that disregards hours of work), and assigned resources using defined calendars for the project and/or resources.

Users wishing to create a manual task list, without utilizing the wonderful dynamic scheduling engine, are better off using Excel. Otherwise, they will fight MS Project from beginning to end because a manually entered start or finish date acts as a 'stake in the ground' around which the rest of schedule pivots seemingly out of control.

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