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In Ganib the Plan dates are set at the time of Project creation. For instance, if you have set Project Start Date as 20 Nov 2014, the Plan Start date will also be 20 Nov 2014.
If you create a Task and set its Start date prior to the Plan Start date (say 18 Nov 2014), this popup will appear. If you click on OK, it will shift your Task Start date but not the Plan Start Date. Those changes will be limited only upto that task.
To know how to change or modify the Plan Start dates, Please refer below link:
If you are facing any other issues, please feel free to contact us. We are happy to help you.