diff --git a/src/lib/insights/derived/__tests__/adherence-storyline-engine-parity.test.ts b/src/lib/insights/derived/__tests__/adherence-storyline-engine-parity.test.ts
index ff5fc0e81..c8c43c269 100644
--- a/src/lib/insights/derived/__tests__/adherence-storyline-engine-parity.test.ts
+++ b/src/lib/insights/derived/__tests__/adherence-storyline-engine-parity.test.ts
@@ -194,3 +194,20 @@ describe("adherence storyline ↔ dashboard-tile engine parity", () => {
     expect(story).toBeNull();
   });
 });
+
+describe("adherence storyline course window", () => {
+  it("keeps a course on its last day in the user's zone", async () => {
+    // 2026-06-21 20:00 in Los Angeles, already the 22nd in UTC. A course
+    // whose `endsOn` (a calendar date) is the 21st is still running.
+    const now = new Date("2026-06-22T03:00:00.000Z");
+    await buildAdherenceStoryline("user-1", "America/Los_Angeles", now);
+    const where = mocks.medicationFindMany.mock.calls[0]![0].where as {
+      OR: [unknown, { endsOn: { gte: Date } }];
+    };
+    const lastDay = new Date("2026-06-21T00:00:00.000Z"); // @db.Date value
+    expect(where.OR[1].endsOn.gte.getTime()).toBeLessThanOrEqual(
+      lastDay.getTime(),
+    );
+    expect(where.OR[1].endsOn.gte.toISOString()).toBe(lastDay.toISOString());
+  });
+});
diff --git a/src/lib/insights/derived/adherence-storyline.ts b/src/lib/insights/derived/adherence-storyline.ts
index 893ff68f0..fd06e1a8b 100644
--- a/src/lib/insights/derived/adherence-storyline.ts
+++ b/src/lib/insights/derived/adherence-storyline.ts
@@ -28,6 +28,7 @@ import { probeRollupCoverage } from "@/lib/rollups/measurement-coverage";
 import { readDayMeanSeries } from "@/lib/insights/derived/baseline";
 import { buildScheduleAnchoredComplianceBuckets } from "@/lib/analytics/schedule-anchored-compliance";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/resolver";
+import { userDayKey } from "@/lib/tz/format";
 import { sanitizeForPrompt } from "@/lib/insights/sanitize";
 import {
   inferMedTargetClass,
@@ -35,6 +36,7 @@ import {
   type MedTargetClass,
 } from "@/lib/medications/med-target-map";
 import { TRACKED_INTAKE_WHERE } from "@/lib/medications/intake-tracking";
+import { dbDate } from "@/lib/tz/date-only";
 
 const MS_PER_DAY = 24 * 60 * 60 * 1000;
 const MAD_TO_SIGMA = 1.4826;
@@ -160,7 +162,11 @@ export async function buildAdherenceStoryline(
   timezone: string | null,
   now: Date = new Date(),
 ): Promise<AdherenceStoryline | null> {
-  // 1. Active medications with a confidently-known target class.
+  // 1. Active medications with a confidently-known target class. `endsOn` is
+  //    a calendar date, so a course is over only once the user's own day has
+  //    passed it; comparing the column against the instant `now` dropped the
+  //    course's last day.
+  const todayKey = userDayKey(now, timezone ?? DEFAULT_TIMEZONE);
   const meds = await prisma.medication.findMany({
     where: {
       userId,
@@ -168,7 +174,7 @@ export async function buildAdherenceStoryline(
       // v1.39.1 (#1033) — the adherence figure below counts tracked
       // medications only, so a record-only one cannot be its subject.
       ...TRACKED_INTAKE_WHERE,
-      OR: [{ endsOn: null }, { endsOn: { gte: now } }],
+      OR: [{ endsOn: null }, { endsOn: { gte: dbDate(todayKey) } }],
     },
     select: { name: true, treatmentClass: true },
   });
diff --git a/src/lib/tz/date-only.ts b/src/lib/tz/date-only.ts
index df98246d9..92526a625 100644
--- a/src/lib/tz/date-only.ts
+++ b/src/lib/tz/date-only.ts
@@ -34,3 +34,13 @@ export function dateOnlyAtNoonUtc(key: string): Date {
 export function dateOnlyKey(value: Date): string {
   return value.toISOString().slice(0, 10);
 }
+
+/**
+ * The value a `@db.Date` column holds for calendar date `key` (Prisma reads
+ * and writes those columns as UTC midnight). Use it to compare such a column
+ * with a day: `endsOn >= dbDate(todayKey)` keeps a course through its last
+ * day, where comparing against the instant `now` dropped that day.
+ */
+export function dbDate(key: string): Date {
+  return new Date(`${key}T00:00:00.000Z`);
+}