diff --git a/api/app_analytics/analytics_db_service.py b/api/app_analytics/analytics_db_service.py
index dd5f336daf1a..8032ee3a303e 100644
--- a/api/app_analytics/analytics_db_service.py
+++ b/api/app_analytics/analytics_db_service.py
@@ -28,6 +28,7 @@
 from app_analytics.types import Labels, PeriodType
 from environments.models import Environment
 from features.models import Feature
+from organisations.billing_periods import months_elapsed, period_start
 from organisations.models import Organisation, OrganisationSubscriptionInformationCache
 
 logger = structlog.get_logger("app_analytics")
@@ -341,9 +342,7 @@ def _get_start_date_and_stop_date_for_subscribed_organisation(
             else:
                 raise NotFound("No billing periods found for this organisation.")
 
-            month_delta = relativedelta(now, starts_at).months
-            date_start = relativedelta(months=month_delta) + starts_at
-            return date_start, now
+            return period_start(starts_at, now), now
 
         case constants.PREVIOUS_BILLING_PERIOD:
             if sub_cache and sub_cache.current_billing_term_starts_at:
@@ -351,11 +350,13 @@ def _get_start_date_and_stop_date_for_subscribed_organisation(
             else:
                 raise NotFound("No billing periods found for this organisation.")
 
-            month_delta = relativedelta(now, starts_at).months - 1
-            month_delta += relativedelta(now, starts_at).years * 12
-            date_start = relativedelta(months=month_delta) + starts_at
-            date_stop = relativedelta(months=month_delta + 1) + starts_at
-            return date_start, date_stop
+            # Both ends count from the term start, so a month too short
+            # for its day does not shift the window.
+            months = months_elapsed(starts_at, now)
+            return (
+                starts_at + relativedelta(months=months - 1),
+                starts_at + relativedelta(months=months),
+            )
 
         case constants.NINETY_DAY_PERIOD:
             date_start = now - relativedelta(days=90)
diff --git a/api/organisations/billing_periods.py b/api/organisations/billing_periods.py
new file mode 100644
index 000000000000..2370e3d2da3c
--- /dev/null
+++ b/api/organisations/billing_periods.py
@@ -0,0 +1,20 @@
+from datetime import datetime
+
+from dateutil.relativedelta import relativedelta
+
+
+def months_elapsed(since: datetime, now: datetime) -> int:
+    """Whole months between two datetimes, years included."""
+    elapsed = relativedelta(now, since)
+    return elapsed.years * 12 + elapsed.months
+
+
+def period_start(billing_term_starts_at: datetime, now: datetime) -> datetime:
+    """
+    Start of the monthly allowance window a term is currently in. A term can
+    run longer than a month, so this is the most recent monthly anniversary of
+    its start.
+    """
+    return billing_term_starts_at + relativedelta(
+        months=months_elapsed(billing_term_starts_at, now)
+    )
diff --git a/api/organisations/models.py b/api/organisations/models.py
index a265f11df649..72e0f82e21a4 100644
--- a/api/organisations/models.py
+++ b/api/organisations/models.py
@@ -25,6 +25,7 @@
 from integrations.lead_tracking.hubspot.tasks import (
     track_hubspot_lead_v2,
 )
+from organisations.billing_periods import months_elapsed
 from organisations.chargebee import (  # type: ignore[attr-defined]
     get_customer_id_from_subscription_id,
     get_max_api_calls_for_plan,
@@ -632,8 +633,7 @@ def current_billing_period(self) -> tuple[datetime, datetime] | None:
         if starts_at is None or not self.has_active_billing_periods():
             return None
 
-        elapsed = relativedelta(timezone.now(), starts_at)
-        months = elapsed.years * 12 + elapsed.months
+        months = months_elapsed(starts_at, timezone.now())
         # Both ends count from the term start. Counting the second from the
         # first loses the original day when a month is too short for it.
         return (
diff --git a/api/organisations/task_helpers.py b/api/organisations/task_helpers.py
index ac0b675f2b5e..211114c8e61c 100644
--- a/api/organisations/task_helpers.py
+++ b/api/organisations/task_helpers.py
@@ -1,7 +1,6 @@
 from datetime import timedelta
 
 import structlog
-from dateutil.relativedelta import relativedelta
 from django.conf import settings
 from django.core.mail import send_mail
 from django.template.loader import render_to_string
@@ -11,6 +10,7 @@
 from app_analytics.influxdb_wrapper import get_current_api_usage
 from core.helpers import get_current_site_url
 from integrations.flagsmith.client import get_openfeature_client
+from organisations.billing_periods import period_start
 from organisations.models import (
     Organisation,
     OrganisationAPIUsageNotification,
@@ -123,9 +123,7 @@ def handle_api_usage_notification_for_organisation(organisation: Organisation) -
             )
             return
 
-        # Truncate to the closest active month to get start of current period.
-        month_delta = _get_total_months(relativedelta(now, billing_starts_at))
-        period_starts_at = relativedelta(months=month_delta) + billing_starts_at
+        period_starts_at = period_start(billing_starts_at, now)
 
         allowed_api_calls = subscription_cache.allowed_30d_api_calls
 
@@ -182,7 +180,3 @@ def handle_api_usage_notification_for_organisation(organisation: Organisation) -
     )
 
     _send_api_usage_notification(organisation, matched_threshold)
-
-
-def _get_total_months(rd: relativedelta) -> int:
-    return rd.months + rd.years * 12
diff --git a/api/organisations/views.py b/api/organisations/views.py
index ee6068272a8d..7049dfd49827 100644
--- a/api/organisations/views.py
+++ b/api/organisations/views.py
@@ -4,7 +4,6 @@
 import logging
 from datetime import timedelta
 
-from dateutil.relativedelta import relativedelta
 from django.utils import timezone
 from drf_spectacular.utils import extend_schema, extend_schema_view
 from rest_framework import status, viewsets
@@ -23,6 +22,7 @@
 )
 from app_analytics.throttles import InfluxQueryThrottle
 from core.helpers import get_current_site_url
+from organisations.billing_periods import period_start
 from organisations.chargebee import webhook_event_types, webhook_handlers
 from organisations.exceptions import OrganisationHasNoPaidSubscription
 from organisations.models import (
@@ -393,8 +393,7 @@ def get_queryset(self):  # type: ignore[no-untyped-def]
         # by defaulting to something as a reasonable default.
         billing_starts_at = billing_starts_at or now - timedelta(days=30)
 
-        month_delta = relativedelta(now, billing_starts_at).months
-        period_starts_at = relativedelta(months=month_delta) + billing_starts_at
+        period_starts_at = period_start(billing_starts_at, now)
 
         queryset = OrganisationAPIUsageNotification.objects.filter(
             organisation_id=organisation.id,
diff --git a/api/tests/unit/app_analytics/test_analytics_db_service.py b/api/tests/unit/app_analytics/test_analytics_db_service.py
index 50321bdcc47a..887a5d6963af 100644
--- a/api/tests/unit/app_analytics/test_analytics_db_service.py
+++ b/api/tests/unit/app_analytics/test_analytics_db_service.py
@@ -7,6 +7,7 @@
 from rest_framework.exceptions import NotFound
 
 from app_analytics.analytics_db_service import (
+    _get_start_date_and_stop_date_for_subscribed_organisation,
     get_feature_evaluation_data,
     get_feature_evaluation_data_from_local_db,
     get_top_organisations_from_local_db,
@@ -966,3 +967,40 @@ def test_get_usage_data_for_window__no_analytics_configured__returns_empty(
 
     # Then
     assert result == []
+
+
+# A term over a year old resolved to the wrong year before #6099, so the current
+# period opened twelve months early and the usage shown was not the period's.
+@pytest.mark.freeze_time("2026-09-11T00:00:00+00:00")
+@pytest.mark.parametrize(
+    "period, expected_start",
+    [
+        (CURRENT_BILLING_PERIOD, "2026-09-07T00:00:00+00:00"),
+        (PREVIOUS_BILLING_PERIOD, "2026-08-07T00:00:00+00:00"),
+    ],
+)
+def test_get_start_date_and_stop_date__term_over_a_year_old__counts_the_years(
+    db: None,
+    organisation: Organisation,
+    period: PeriodType,
+    expected_start: str,
+) -> None:
+    # Given
+    sub_cache = OrganisationSubscriptionInformationCache.objects.create(
+        organisation=organisation,
+        current_billing_term_starts_at=datetime.fromisoformat(
+            "2025-08-07T00:00:00+00:00"
+        ),
+        current_billing_term_ends_at=datetime.fromisoformat(
+            "2027-08-07T00:00:00+00:00"
+        ),
+    )
+
+    # When
+    date_start, _ = _get_start_date_and_stop_date_for_subscribed_organisation(
+        sub_cache=sub_cache,
+        period=period,
+    )
+
+    # Then
+    assert date_start == datetime.fromisoformat(expected_start)
diff --git a/api/tests/unit/organisations/test_unit_organisations_billing_periods.py b/api/tests/unit/organisations/test_unit_organisations_billing_periods.py
new file mode 100644
index 000000000000..68f24b1c6562
--- /dev/null
+++ b/api/tests/unit/organisations/test_unit_organisations_billing_periods.py
@@ -0,0 +1,54 @@
+from datetime import datetime
+
+import pytest
+
+from organisations.billing_periods import months_elapsed, period_start
+
+
+@pytest.mark.parametrize(
+    "since, now, expected",
+    [
+        ("2026-09-01T00:00:00+00:00", "2026-09-10T00:00:00+00:00", 0),
+        ("2026-01-05T00:00:00+00:00", "2026-09-10T00:00:00+00:00", 8),
+        # The year is the part #6099 dropped.
+        ("2024-09-03T00:00:00+00:00", "2026-09-10T00:00:00+00:00", 24),
+        ("2025-08-07T00:00:00+00:00", "2026-09-11T00:00:00+00:00", 13),
+    ],
+)
+def test_months_elapsed__spans_years__counts_them(
+    since: str, now: str, expected: int
+) -> None:
+    # Given / When
+    elapsed = months_elapsed(datetime.fromisoformat(since), datetime.fromisoformat(now))
+
+    # Then
+    assert elapsed == expected
+
+
+@pytest.mark.parametrize(
+    "term_starts_at, now, expected",
+    [
+        # Annual term over a year old. Dropping the year lands in 2025.
+        (
+            "2024-09-03T00:00:00+00:00",
+            "2026-09-10T00:00:00+00:00",
+            "2026-09-03T00:00:00+00:00",
+        ),
+        # February is too short for a 31st, so the window opens on the 28th.
+        (
+            "2026-01-31T00:00:00+00:00",
+            "2026-03-01T00:00:00+00:00",
+            "2026-02-28T00:00:00+00:00",
+        ),
+    ],
+)
+def test_period_start__long_term__opens_on_the_latest_anniversary(
+    term_starts_at: str, now: str, expected: str
+) -> None:
+    # Given / When
+    start = period_start(
+        datetime.fromisoformat(term_starts_at), datetime.fromisoformat(now)
+    )
+
+    # Then
+    assert start == datetime.fromisoformat(expected)
diff --git a/api/tests/unit/organisations/test_unit_organisations_views.py b/api/tests/unit/organisations/test_unit_organisations_views.py
index 062ea3bba09c..062bf2b9e3fd 100644
--- a/api/tests/unit/organisations/test_unit_organisations_views.py
+++ b/api/tests/unit/organisations/test_unit_organisations_views.py
@@ -2234,3 +2234,47 @@ def test_get_detailed_permissions__other_user_as_admin__returns_permissions(
             "derived_from": {"groups": [], "roles": []},
         }
     ]
+
+
+# A term over a year old resolved to the wrong year before #6099, so the window
+# opened twelve months early and swept up notifications from previous periods.
+@pytest.mark.freeze_time("2026-09-11T00:00:00+00:00")
+def test_get_api_usage_notifications__term_over_a_year_old__excludes_earlier_periods(
+    staff_client: APIClient,
+    organisation: Organisation,
+) -> None:
+    # Given
+    now = timezone.now()
+    OrganisationSubscriptionInformationCache.objects.create(
+        organisation=organisation,
+        current_billing_term_starts_at=datetime.fromisoformat(
+            "2025-08-07T00:00:00+00:00"
+        ),
+        current_billing_term_ends_at=datetime.fromisoformat(
+            "2027-08-07T00:00:00+00:00"
+        ),
+    )
+    # Inside the current window, which opens on 7 September 2026.
+    OrganisationAPIUsageNotification.objects.create(
+        organisation=organisation,
+        percent_usage=90,
+        notified_at=now,
+    )
+    # A year earlier, only reachable if the year is dropped.
+    OrganisationAPIUsageNotification.objects.create(
+        organisation=organisation,
+        percent_usage=100,
+        notified_at=datetime.fromisoformat("2025-09-20T00:00:00+00:00"),
+    )
+
+    url = reverse(
+        "api-v1:organisations:organisation-api-usage-notification",
+        args=[organisation.id],
+    )
+
+    # When
+    response = staff_client.get(url)
+
+    # Then
+    assert response.status_code == status.HTTP_200_OK
+    assert [r["percent_usage"] for r in response.data["results"]] == [90]
diff --git a/docs/docs/deployment-self-hosting/observability/_events-catalogue.md b/docs/docs/deployment-self-hosting/observability/_events-catalogue.md
index 3826b5823873..7c020d6d1cac 100644
--- a/docs/docs/deployment-self-hosting/observability/_events-catalogue.md
+++ b/docs/docs/deployment-self-hosting/observability/_events-catalogue.md
@@ -2,7 +2,7 @@
 ### `api_usage.notification.evaluated`
 
 Logged at `info` from:
- - `api/organisations/task_helpers.py:155`
+ - `api/organisations/task_helpers.py:153`
 
 Attributes:
  - `allowed_api_calls`
@@ -24,7 +24,7 @@ Attributes:
 ### `api_usage.notification.sent`
 
 Logged at `info` from:
- - `api/organisations/task_helpers.py:178`
+ - `api/organisations/task_helpers.py:176`
 
 Attributes:
  - `matched_threshold`
@@ -33,9 +33,9 @@ Attributes:
 ### `app_analytics.no_analytics_database_configured`
 
 Logged at `warning` from:
- - `api/app_analytics/analytics_db_service.py:74`
- - `api/app_analytics/analytics_db_service.py:187`
- - `api/app_analytics/analytics_db_service.py:278`
+ - `api/app_analytics/analytics_db_service.py:75`
+ - `api/app_analytics/analytics_db_service.py:188`
+ - `api/app_analytics/analytics_db_service.py:279`
 
 Attributes:
  - `details`