diff --git a/kfet/tests/test_models.py b/kfet/tests/test_models.py
index 7ce6605c..a534493d 100644
--- a/kfet/tests/test_models.py
+++ b/kfet/tests/test_models.py
@@ -1,10 +1,12 @@
-import datetime
+from datetime import datetime, timedelta, timezone as tz
+from decimal import Decimal
+from unittest import mock
 
 from django.contrib.auth import get_user_model
 from django.test import TestCase
 from django.utils import timezone
 
-from kfet.models import Account, Checkout
+from kfet.models import Account, AccountNegative, Checkout
 
 from .utils import create_user
 
@@ -28,6 +30,56 @@ class AccountTests(TestCase):
         with self.assertRaises(Account.DoesNotExist):
             Account.objects.get_by_password("bernard")
 
+    @mock.patch("django.utils.timezone.now")
+    def test_negative_creation(self, mock_now):
+        now = datetime(2005, 7, 15, tzinfo=tz.utc)
+        mock_now.return_value = now
+        self.account.balance = Decimal(-10)
+        self.account.update_negative()
+
+        self.assertTrue(hasattr(self.account, "negative"))
+        self.assertEqual(self.account.negative.start, now)
+
+    @mock.patch("django.utils.timezone.now")
+    def test_negative_no_reset(self, mock_now):
+        now = datetime(2005, 7, 15, tzinfo=tz.utc)
+        mock_now.return_value = now
+
+        self.account.balance = Decimal(-10)
+        AccountNegative.objects.create(
+            account=self.account, start=now - timedelta(minutes=3)
+        )
+        self.account.refresh_from_db()
+
+        self.account.balance = Decimal(5)
+        self.account.update_negative()
+        self.assertTrue(hasattr(self.account, "negative"))
+
+        self.account.balance = Decimal(-10)
+        self.account.update_negative()
+        self.assertEqual(self.account.negative.start, now - timedelta(minutes=3))
+
+    @mock.patch("django.utils.timezone.now")
+    def test_negative_eventually_resets(self, mock_now):
+        now = datetime(2005, 7, 15, tzinfo=tz.utc)
+        mock_now.return_value = now
+
+        self.account.balance = Decimal(-10)
+        AccountNegative.objects.create(
+            account=self.account, start=now - timedelta(minutes=20)
+        )
+        self.account.refresh_from_db()
+        self.account.balance = Decimal(5)
+
+        mock_now.return_value = now - timedelta(minutes=10)
+        self.account.update_negative()
+
+        mock_now.return_value = now
+        self.account.update_negative()
+        self.account.refresh_from_db()
+
+        self.assertFalse(hasattr(self.account, "negative"))
+
 
 class CheckoutTests(TestCase):
     def setUp(self):
@@ -39,7 +91,7 @@ class CheckoutTests(TestCase):
         self.c = Checkout(
             created_by=self.u_acc,
             valid_from=self.now,
-            valid_to=self.now + datetime.timedelta(days=1),
+            valid_to=self.now + timedelta(days=1),
         )
 
     def test_initial_statement(self):