2 files changed, 69 insertions, 1 deletions

diff --git a/src/shared/date.test.ts b/src/shared/date.test.ts
index 1c61a15..da832f5 100644
--- a/src/shared/date.test.ts
+++ b/src/shared/date.test.ts
@@ -1,5 +1,52 @@
 import { describe, expect, it } from "vitest";
-import { getEndOfStudyDayBoundary } from "./date";
+import { getEndOfStudyDayBoundary, getStartOfStudyDayBoundary } from "./date";
+
+describe("getStartOfStudyDayBoundary", () => {
+	it("should return today 3:00 AM when current time is after 3:00 AM", () => {
+		// Feb 2, 2026 10:00 AM
+		const now = new Date(2026, 1, 2, 10, 0, 0, 0);
+		const boundary = getStartOfStudyDayBoundary(now);
+
+		expect(boundary.getFullYear()).toBe(2026);
+		expect(boundary.getMonth()).toBe(1);
+		expect(boundary.getDate()).toBe(2);
+		expect(boundary.getHours()).toBe(3);
+		expect(boundary.getMinutes()).toBe(0);
+		expect(boundary.getSeconds()).toBe(0);
+	});
+
+	it("should return yesterday 3:00 AM when current time is before 3:00 AM", () => {
+		// Feb 2, 2026 1:30 AM
+		const now = new Date(2026, 1, 2, 1, 30, 0, 0);
+		const boundary = getStartOfStudyDayBoundary(now);
+
+		expect(boundary.getFullYear()).toBe(2026);
+		expect(boundary.getMonth()).toBe(1);
+		expect(boundary.getDate()).toBe(1);
+		expect(boundary.getHours()).toBe(3);
+		expect(boundary.getMinutes()).toBe(0);
+		expect(boundary.getSeconds()).toBe(0);
+	});
+
+	it("should return today 3:00 AM when current time is exactly 3:00 AM", () => {
+		// Feb 2, 2026 3:00 AM
+		const now = new Date(2026, 1, 2, 3, 0, 0, 0);
+		const boundary = getStartOfStudyDayBoundary(now);
+
+		expect(boundary.getDate()).toBe(2);
+		expect(boundary.getHours()).toBe(3);
+	});
+
+	it("should handle month boundaries correctly", () => {
+		// Feb 1, 2026 1:00 AM
+		const now = new Date(2026, 1, 1, 1, 0, 0, 0);
+		const boundary = getStartOfStudyDayBoundary(now);
+
+		expect(boundary.getMonth()).toBe(0); // January
+		expect(boundary.getDate()).toBe(31);
+		expect(boundary.getHours()).toBe(3);
+	});
+});
 
 describe("getEndOfStudyDayBoundary", () => {
 	it("should return next day 3:00 AM when current time is after 3:00 AM", () => {
diff --git a/src/shared/date.ts b/src/shared/date.ts
index 583d2a6..3b34f14 100644
--- a/src/shared/date.ts
+++ b/src/shared/date.ts
@@ -1,4 +1,25 @@
 /**
+ * Returns the start-of-day boundary for the current study day.
+ *
+ * The "study day" is defined as 3:00 AM to the next day's 3:00 AM.
+ *
+ * - If current time >= 3:00 AM, start = today 3:00 AM local time
+ * - If current time < 3:00 AM, start = yesterday 3:00 AM local time
+ */
+export function getStartOfStudyDayBoundary(now: Date = new Date()): Date {
+	const boundary = new Date(now);
+	boundary.setMinutes(0, 0, 0);
+
+	if (boundary.getHours() < 3) {
+		// Move to previous day
+		boundary.setDate(boundary.getDate() - 1);
+	}
+
+	boundary.setHours(3);
+	return boundary;
+}
+
+/**
  * Returns the end-of-day boundary for due card comparison.
  *
  * The "study day" is defined as 3:00 AM to the next day's 3:00 AM.