fix: prorate factor for subscription plan (#36953)

(cherry picked from commit fc79af5926)

erpnext/accounts/doctype/subscription/test_subscription.py

@@ -694,3 +694,23 @@ class TestSubscription(unittest.TestCase):
 		# Check the currency of the created invoice
 		currency = frappe.db.get_value("Sales Invoice", subscription.invoices[0].invoice, "currency")
 		self.assertEqual(currency, "USD")
+
+	def test_plan_rate_for_midmonth_start_date(self):
+		subscription = frappe.new_doc("Subscription")
+		subscription.party_type = "Supplier"
+		subscription.party = "_Test Supplier"
+		subscription.generate_invoice_at_period_start = 1
+		subscription.follow_calendar_months = 1
+		subscription.generate_new_invoices_past_due_date = 1
+		subscription.start_date = "2023-04-08"
+		subscription.end_date = "2024-02-27"
+		subscription.append("plans", {"plan": "_Test Plan Name 4", "qty": 1})
+		subscription.save()
+
+		subscription.process()
+
+		self.assertEqual(len(subscription.invoices), 1)
+		pi = frappe.get_doc("Purchase Invoice", subscription.invoices[0].invoice)
+		self.assertEqual(pi.total, 55333.33)
+
+		subscription.delete()

erpnext/accounts/doctype/subscription_plan/subscription_plan.py

@@ -56,18 +56,17 @@ def get_plan_rate(
 		prorate = frappe.db.get_single_value("Subscription Settings", "prorate")
 
 		if prorate:
-			prorate_factor = flt(
-				date_diff(start_date, get_first_day(start_date))
-				/ date_diff(get_last_day(start_date), get_first_day(start_date)),
-				1,
-			)
-
-			prorate_factor += flt(
-				date_diff(get_last_day(end_date), end_date)
-				/ date_diff(get_last_day(end_date), get_first_day(end_date)),
-				1,
-			)
-
-			cost -= plan.cost * prorate_factor
-
+			cost -= plan.cost * get_prorate_factor(start_date, end_date)
 		return cost
+
+
+def get_prorate_factor(start_date, end_date):
+	total_days_to_skip = date_diff(start_date, get_first_day(start_date))
+	total_days_in_month = int(get_last_day(start_date).strftime("%d"))
+	prorate_factor = flt(total_days_to_skip / total_days_in_month)
+
+	total_days_to_skip = date_diff(get_last_day(end_date), end_date)
+	total_days_in_month = int(get_last_day(end_date).strftime("%d"))
+	prorate_factor += flt(total_days_to_skip / total_days_in_month)
+
+	return prorate_factor