oca-project/odoo-bringout-oca-project-project_timeline/project_timeline/demo/project_project_demo.xml

<?xml version="1.0" encoding="utf-8" ?>
<!-- Copyright 2018 Onestein
     License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl). -->
<odoo>
    <record id="project.project_project_1" model="project.project">
        <field
            name="date_start"
            eval="(DateTime.today() - timedelta(days=5)).strftime('%Y-%m-%d')"
        />
        <field
            name="date"
            eval="(DateTime.today() + timedelta(days=2)).strftime('%Y-%m-%d')"
        />
    </record>
    <record id="project.project_project_2" model="project.project">
        <field
            name="date_start"
            eval="(DateTime.today() - timedelta(days=30)).strftime('%Y-%m-%d')"
        />
        <field
            name="date"
            eval="(DateTime.today() + timedelta(days=10)).strftime('%Y-%m-%d')"
        />
    </record>
</odoo>