I'm trying to add a new field "End date" in project.task.work that would be related to 'date' (start date) and 'duration'. That seems to be ok. But when I'm customizing the view project.view_task_form2 to add my new field end_date, I got a error of type ViewError:

Can't find field 'end_date' in the following view parts composing the view of object model 'project.task':
 * project.task.form.inherit

Either you wrongly customized this view, or some modules bringing those views are not compatible with your current data model

Here is the code:

class CustomProjectWork(osv.osv):
    _name = "project.task.work"
    _inherit = "project.task.work"

    def _determin_end_date(self, cr, uid, ids, field, arg, context=None):
        result = {}
        for projectwork in self.browse(cr, uid, ids, context=context):
            if projectwork.date and projectwork.hours:
                # Get a datetime object from projectwork.date
                start_date = datetime.strptime(projectwork.date, "%Y-%m-%d %H:%M:%S")
                # Get a timedelta object from projectwork.duration
                duration = timedelta(days=(projectwork.duration - 1))
                # Calculate the end time
                end_date = start_date + duration
                # Render in format YYYY-MM-DD
                result[projectwork.id] = end_date.strftime("%Y-%m-%d %H:%M:%S")
            else:
                result[projectwork.id] = projectwork.date
                return result

    def _set_end_date(self, cr, uid, id, field, value, arg, context=None):
        projectwork = self.browse(cr, uid, id, context=context)
        if projectwork.date and value:
            start_date = datetime.strptime(projectwork.date, "%Y-%m-%d %H:%M:%S")
            end_date = datetime.strptime(value[:20], "%Y-%m-%d %H:%M:%S")
            duration = end_date - start_date
            self.write(cr, uid, id, {'duration': (duration.days + 1)}, context=context)

    _column = {
        'end_date': fields.function(_determin_end_date, fnct_inv=_set_end_date, type='datetime', string="End Date", store=True)
    }

The view:

<record id="view_task_form2_inherit" model="ir.ui.view">
            <field name="name">project.task.form.inherit</field>
            <field name="model">project.task</field>
            <field name="inherit_id" ref="project.view_task_form2" />
            <field name="type">form</field>
            <field name="arch" type="xml">
                <field name="work_ids" position="replace">
                    <field colspan="4" name="work_ids" nolabel="1" attrs="{'readonly':[('state','in',['draft','done'])]}">
                        <tree string="Task Work" editable="top">
                            <field name="date" />
                            <field name="end_date" />
                            <field name="user_id" />
                            <field name="hours" widget="float_time" />
                            <field name="name" />
                        </tree>
                    </field>
                </field>
            </field>
</record>