Follow these steps to add a calendar view for the project.task model:

1. Define a calendar view:

<record id="view_project_task_calendar" model="ir.ui.view">
    <field name="model">project.task</field>
    <field name="arch" type="xml">
        <calendar date_start="date_start" date_stop="date_end" color="project_id">
            <field name="name" />
            <field name="user_id" />
        </calendar>
    </field>
</record>

2. Add menus and actions using this view. This is left as an exercise for the reader.