In Microsoft Project, when I update the % complete, it changes my start/finish times. Why?

There's a long answer, and a short answer.

The long answer involves discussing how you have set up your project tasks and how you are entering your progress information. This problem is probably symptomatic of other issues that may need to be addressed. It also depends on what Calculation options you have set Tools->Options->Calculation

The short answer is that % Complete in MS Project is Schedule Percent Complete. If you started two weeks ago and are supposed to be finished tomorrow, but your report only being 50% done, then Project assumes you need another 2 weeks to get the job done.

To avoid this, don't use the % Complete field but insert the Physical Percent Complete field/column into your view and use that instead.

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