fix(study): use date-based comparison with 3 AM boundary for due cards

Instead of comparing due timestamps exactly (card.due <= now), compare
against the next 3 AM boundary so all cards due within the current
study day appear at once. This prevents new cards from trickling in
throughout the day when FSRS fuzz spreads due times.

https://claude.ai/code/session_01FeDztLcyGofd6nxh8ct7a3

1 files changed, 58 insertions, 0 deletions

diff --git a/src/shared/date.test.ts b/src/shared/date.test.ts
new file mode 100644
index 0000000..1c61a15
--- /dev/null
+++ b/src/shared/date.test.ts
@@ -0,0 +1,58 @@
+import { describe, expect, it } from "vitest";
+import { getEndOfStudyDayBoundary } from "./date";
+
+describe("getEndOfStudyDayBoundary", () => {
+	it("should return next day 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 = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getFullYear()).toBe(2026);
+		expect(boundary.getMonth()).toBe(1);
+		expect(boundary.getDate()).toBe(3);
+		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 before 3:00 AM", () => {
+		// Feb 2, 2026 1:30 AM
+		const now = new Date(2026, 1, 2, 1, 30, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(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 next day 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 = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getDate()).toBe(3);
+		expect(boundary.getHours()).toBe(3);
+	});
+
+	it("should return next day 3:00 AM when current time is 11:59 PM", () => {
+		// Feb 2, 2026 11:59 PM
+		const now = new Date(2026, 1, 2, 23, 59, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getDate()).toBe(3);
+		expect(boundary.getHours()).toBe(3);
+	});
+
+	it("should handle month boundaries correctly", () => {
+		// Jan 31, 2026 15:00
+		const now = new Date(2026, 0, 31, 15, 0, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getMonth()).toBe(1); // February
+		expect(boundary.getDate()).toBe(1);
+		expect(boundary.getHours()).toBe(3);
+	});
+});