diff --git a/src/app/api/dashboard/activity/route.ts b/src/app/api/dashboard/activity/route.ts
index 20371f7..1c28b5c 100644
--- a/src/app/api/dashboard/activity/route.ts
+++ b/src/app/api/dashboard/activity/route.ts
@@ -16,8 +16,8 @@ import MealPlan from "@/lib/models/meal-plan";
 import Expense from "@/lib/models/expense";
 import CustomEntry from "@/lib/models/custom-entry";
 import SectionTemplate from "@/lib/models/section-template";
-import { startOfMonth, endOfMonth, format } from "date-fns";
 import { DEFAULT_ENABLED_SECTIONS, type SectionId } from "@/lib/constants";
+import { toDateKey, utcMonthRange } from "@/lib/date-key";
 
 export async function GET(req: NextRequest) {
   const session = await auth();
@@ -31,15 +31,7 @@ export async function GET(req: NextRequest) {
   const { searchParams } = new URL(req.url);
   const monthParam = searchParams.get("month"); // "2026-05"
 
-  let start: Date, end: Date;
-  if (monthParam) {
-    const d = new Date(monthParam + "-01");
-    start = startOfMonth(d);
-    end = endOfMonth(d);
-  } else {
-    start = startOfMonth(new Date());
-    end = endOfMonth(new Date());
-  }
+  const { start, end } = utcMonthRange(monthParam ? new Date(monthParam + "-01") : new Date());
 
   const user = await User.findById(userId).lean();
   if (!user) {
@@ -54,7 +46,7 @@ export async function GET(req: NextRequest) {
 
   const addDates = (dates: Date[], sectionId: string) => {
     for (const d of dates) {
-      const key = format(d, "yyyy-MM-dd");
+      const key = toDateKey(d);
       if (!activity[key]) activity[key] = [];
       if (!activity[key].includes(sectionId)) activity[key].push(sectionId);
     }
@@ -154,7 +146,7 @@ export async function GET(req: NextRequest) {
           for (const doc of docs) {
             const slug = templateMap.get(String(doc.templateId));
             if (slug) {
-              const key = format(doc.date, "yyyy-MM-dd");
+              const key = toDateKey(doc.date);
               if (!activity[key]) activity[key] = [];
               if (!activity[key].includes(slug)) activity[key].push(slug);
             }
diff --git a/src/lib/__tests__/date-key.test.ts b/src/lib/__tests__/date-key.test.ts
new file mode 100644
index 0000000..0837c5f
--- /dev/null
+++ b/src/lib/__tests__/date-key.test.ts
@@ -0,0 +1,51 @@
+import { describe, it, expect, afterEach } from "vitest";
+import { toDateKey, utcMonthRange } from "../date-key";
+
+describe("toDateKey", () => {
+  const originalTz = process.env.TZ;
+
+  afterEach(() => {
+    process.env.TZ = originalTz;
+  });
+
+  it("returns the intended calendar day regardless of the server process's local timezone", () => {
+    // Date-only fields are stored as UTC midnight of the intended day (e.g.
+    // "2026-01-15" parses to 2026-01-15T00:00:00.000Z). A server running in a
+    // negative-UTC-offset timezone must not read this back as the prior day.
+    const utcMidnight = new Date("2026-01-15T00:00:00.000Z");
+
+    process.env.TZ = "America/New_York"; // UTC-5 in January
+    expect(toDateKey(utcMidnight)).toBe("2026-01-15");
+
+    process.env.TZ = "Pacific/Kiritimati"; // UTC+14 — the other extreme
+    expect(toDateKey(utcMidnight)).toBe("2026-01-15");
+
+    process.env.TZ = "UTC";
+    expect(toDateKey(utcMidnight)).toBe("2026-01-15");
+  });
+});
+
+describe("utcMonthRange", () => {
+  const originalTz = process.env.TZ;
+
+  afterEach(() => {
+    process.env.TZ = originalTz;
+  });
+
+  it("spans the intended UTC month regardless of the server process's local timezone", () => {
+    // "2026-01-01" parses to UTC midnight Jan 1. date-fns' startOfMonth/
+    // endOfMonth (local-timezone-based) would shift this range onto December
+    // on a server running in a negative-UTC-offset timezone like EST.
+    const anchor = new Date("2026-01-01T00:00:00.000Z");
+
+    process.env.TZ = "America/New_York";
+    let { start, end } = utcMonthRange(anchor);
+    expect(start.toISOString()).toBe("2026-01-01T00:00:00.000Z");
+    expect(end.toISOString()).toBe("2026-01-31T23:59:59.999Z");
+
+    process.env.TZ = "Pacific/Kiritimati";
+    ({ start, end } = utcMonthRange(anchor));
+    expect(start.toISOString()).toBe("2026-01-01T00:00:00.000Z");
+    expect(end.toISOString()).toBe("2026-01-31T23:59:59.999Z");
+  });
+});
diff --git a/src/lib/date-key.ts b/src/lib/date-key.ts
new file mode 100644
index 0000000..0b0cd9f
--- /dev/null
+++ b/src/lib/date-key.ts
@@ -0,0 +1,26 @@
+/**
+ * Converts a Date to its "yyyy-MM-dd" calendar-day key using UTC getters.
+ *
+ * Date-only fields throughout this app are stored as UTC midnight of the
+ * intended calendar day (e.g. the string "2026-01-15" parses to
+ * 2026-01-15T00:00:00.000Z). Reading such a value back with local-timezone
+ * getters (as date-fns' format() does) can shift the key by a day whenever
+ * the process isn't running in UTC. Using UTC getters keeps the key
+ * independent of the server's local timezone.
+ */
+export function toDateKey(d: Date): string {
+  return d.toISOString().slice(0, 10);
+}
+
+/**
+ * Returns the [start, end] instants spanning the UTC calendar month
+ * containing `d`. Using date-fns' startOfMonth/endOfMonth here would use the
+ * server process's local timezone, which can shift the queried range by a
+ * day (or onto the wrong month entirely) relative to UTC-anchored dates.
+ */
+export function utcMonthRange(d: Date): { start: Date; end: Date } {
+  return {
+    start: new Date(Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), 1)),
+    end: new Date(Date.UTC(d.getUTCFullYear(), d.getUTCMonth() + 1, 0, 23, 59, 59, 999)),
+  };
+}