Hi,

For this you can add a validation on creating the leave request. And you can check whether the difference between current date and requested leave date,. If it is not 15, you can show a warning message else accept the leave request.

Otherwise you can add a automated action to check and refuse the leave, based on the create date and requested leave date.

Also you can check with this community module for the auto leave approval, making slight changes in the code may help you to achieve what you are looking for.

https://www.odoo.com/apps/modules/9.0/hr_holidays_leave_auto_approve/

Thanks