Thanks @Cybrosys, but that did not work for me. 

Just as a reference, I solved the problem by:

Changing the default value from "Waiting for another wo" to "ready" and 
 redefining the following compute methods

date_planned_start = fields.Datetime(
        compute=lambda self: self._compute_date_planned_start(),
        default=lambda self: self._default_date_planned_start(),
        string='Planned Start',
    )

date_planned_finished = fields.Datetime(
    compute=lambda self: self._compute_date_planned_start(),
    string='Planned End',
)