On newly created projects I can not edit the Planned Cost Start and Planned Cost Finish dates when placed on the project list view. However on ideas I can edit them as I wish.
Those fields are not editable when the Set Planned Cost Dates field is checked.
When the field is checked, the Planned Cost Start and Planned Cost Finish dates will be in sync with the Investment Start and Finish dates.
If the flag is off you can edit the dates as you wish.
On the project object, the Set Planned Cost Dates field boolean field is checked by default.
On the idea object, the Set Planned Cost Dates field boolean field is unchecked by default. This because idea Start and Finish dates are not mandatory by default.
Please note, the behavior changes when detailed financial plans exist. As simple budget data gets overwritten by the detailed financial plans. The Set Planned Cost Dates field is intended for simple budgeting.
If the Set Planned Cost Dates flag is enabled, if no detailed financial plans exist, the Planned Cost Start and Finish dates are set according to the investment start and finish dates. And those dates are not editable.
If the flag is off, you can change the Planned Cost Start and Finish dates accordingly.
When you go to the Financials Plans and create a detailed financial plan, the initial dates get overwritten by the dates on the financial plan.You will also notice that at this stage the Set Planned Cost Dates flag gets disabled.