Hi,

You can define the cron job like this,

In XML

<record id="ir_cron_scheduler_demo_action" model="ir.cron">
            <field name="name">Due Notification</field>
            <field name="user_id" ref="base.user_root"/>
            <field name="interval_number">15</field>
            <field name="interval_type">days</field>
            <field name="numbercall">-1</field>
            <field eval="False" name="doall"/>
            <field eval="'model_name'" name="model"/>
            <field eval="'due_notification'" name="function"/>
        </record>

This function will get triggered in an interval of 15 days.

You didn't mention the the problem that you are facing. Then you have to define  a python function in the name "due_notification" in the corresponding model.

Inside the python function you can write the code for sending the mail like this,

class DueNotification(models.Model):
    _inherit = 'model_name'

    @api.multi
    def due_notification(self):
	    template_obj = self.env['mail.mail']
	    template_data = {
		'subject': 'Due Notification : ',
		'body_html': "Body....",
		'email_from': "test@mail.com",
		'email_to': "me@mail.com"
	    }
	    template_id = template_obj.create(template_data)
	    template_obj.send(template_id)

For getting the date after 7 days,

from time import gmtime, strftime
import datetime from dateutil.relativedelta
import relativedelta

current_date = strftime("%Y-%m-%d %H:%M:%S", gmtime())
date_1 = datetime.strptime(current_date, "%Y-%m-%d %H:%M:%S")
date_after_7_days = date_1 + relativedelta(days=7)

Thanks