The Plan Dates in the Plan Details Page allow you define the Planned Start, Planned End and Revised End dates of the Plan.
The Planned Start date will typically be the date at which its Workflow Status changes to 'Approved' and the Planned End will typically be the date at which its Workflow Status changes to 'Live’, however these conventions are a matter for definition by your organisation. Note that the Planned End date is the original Planned End date; the Revised End date is used to record all subsequent changes to that date.