diff --git a/addons/purchase_stock/models/res_partner.py b/addons/purchase_stock/models/res_partner.py
index 50ae5b9d3ba..dacce687f4a 100644
--- a/addons/purchase_stock/models/res_partner.py
+++ b/addons/purchase_stock/models/res_partner.py
@@ -13,13 +13,15 @@ class ResPartner(models.Model):
     purchase_line_ids = fields.One2many('purchase.order.line', 'partner_id', string="Purchase Lines")
     on_time_rate = fields.Float(
         "On-Time Delivery Rate", compute='_compute_on_time_rate',
-        help="Over the past 12 months; the number of products received on time divided by the number of ordered products.")
+        help="Over the past x days; the number of products received on time divided by the number of ordered products."\
+            "x is either the System Parameter purchase_stock.on_time_delivery_days or the default 365")
 
     @api.depends('purchase_line_ids')
     def _compute_on_time_rate(self):
+        date_order_days_delta = int(self.env['ir.config_parameter'].sudo().get_param('purchase_stock.on_time_delivery_days', default='365'))
         order_lines = self.env['purchase.order.line'].search([
             ('partner_id', 'in', self.ids),
-            ('date_order', '>', fields.Date.today() - timedelta(365)),
+            ('date_order', '>', fields.Date.today() - timedelta(date_order_days_delta)),
             ('qty_received', '!=', 0),
             ('order_id.state', 'in', ['done', 'purchase']),
             ('product_id', 'in', self.env['product.product'].sudo()._search([('type', '!=', 'service')]))