diff --git a/backend/invoices/models.py b/backend/invoices/models.py
index 4c8bcbc6..5e582f2a 100644
--- a/backend/invoices/models.py
+++ b/backend/invoices/models.py
@@ -1264,7 +1264,11 @@ class RecurringInvoice(AssignableMixin, BaseModel):
         return self._add_months(current, 1)
 
     def _add_months(self, current, months):
-        """Add months, restoring the start_date day after a month-end clamp."""
+        """Add months, restoring the start_date day after a month-end clamp.
+
+        start_date.day is the billing anchor: a next_generation_date on the
+        last day of a short month is treated as clamped and moves back to it.
+        """
         import calendar
 
         from dateutil.relativedelta import relativedelta
diff --git a/backend/invoices/tests/test_invoices_api.py b/backend/invoices/tests/test_invoices_api.py
index 66317679..1aecd21d 100644
--- a/backend/invoices/tests/test_invoices_api.py
+++ b/backend/invoices/tests/test_invoices_api.py
@@ -4830,6 +4830,25 @@ class TestRecurringInvoiceModel:
         recurring_invoice.next_generation_date = datetime.date(2026, 2, 15)
         assert recurring_invoice.calculate_next_date() == datetime.date(2026, 3, 15)
 
+    def test_calculate_next_date_month_end_anchors(self, recurring_invoice):
+        d = datetime.date
+        cases = [
+            ("MONTHLY", d(2026, 1, 30), d(2026, 2, 28), d(2026, 3, 30)),
+            ("MONTHLY", d(2026, 1, 31), d(2026, 12, 31), d(2027, 1, 31)),
+            ("QUARTERLY", d(2026, 1, 31), d(2026, 4, 30), d(2026, 7, 31)),
+            ("SEMI_ANNUALLY", d(2026, 8, 31), d(2026, 2, 28), d(2026, 8, 31)),
+            ("MONTHLY", d(2026, 1, 31), d(2026, 2, 28), d(2026, 3, 31)),
+        ]
+        for frequency, start, current, expected in cases:
+            recurring_invoice.frequency = frequency
+            recurring_invoice.start_date = start
+            recurring_invoice.next_generation_date = current
+            assert recurring_invoice.calculate_next_date() == expected, (
+                frequency,
+                start,
+                current,
+            )
+
     def test_calculate_next_date_custom(self, recurring_invoice):
         recurring_invoice.frequency = "CUSTOM"
         recurring_invoice.custom_days = 10