diff --git a/src/engine/datetime.test.ts b/src/engine/datetime.test.ts
new file mode 100644
index 0000000..c327c52
--- /dev/null
+++ b/src/engine/datetime.test.ts
@@ -0,0 +1,208 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import {
+  detectDateOrder,
+  resolveZone,
+  startOfToday,
+  addToDate,
+  isCalendarUnit,
+} from "./datetime";
+
+describe("datetime", () => {
+  describe("detectDateOrder", () => {
+    afterEach(() => {
+      vi.restoreAllMocks();
+    });
+
+    it("should detect dmy", () => {
+      const mockFormatToParts = vi.fn().mockReturnValue([
+        { type: "day", value: "31" },
+        { type: "literal", value: "/" },
+        { type: "month", value: "12" },
+        { type: "literal", value: "/" },
+        { type: "year", value: "2000" },
+      ]);
+
+      const mockDateTimeFormat = vi.spyOn(Intl, "DateTimeFormat").mockImplementation(() => ({
+        formatToParts: mockFormatToParts,
+      }) as any);
+
+      expect(detectDateOrder()).toBe("dmy");
+      mockDateTimeFormat.mockRestore();
+    });
+
+    it("should detect mdy", () => {
+      const mockFormatToParts = vi.fn().mockReturnValue([
+        { type: "month", value: "12" },
+        { type: "literal", value: "/" },
+        { type: "day", value: "31" },
+        { type: "literal", value: "/" },
+        { type: "year", value: "2000" },
+      ]);
+
+      const mockDateTimeFormat = vi.spyOn(Intl, "DateTimeFormat").mockImplementation(() => ({
+        formatToParts: mockFormatToParts,
+      }) as any);
+
+      expect(detectDateOrder()).toBe("mdy");
+      mockDateTimeFormat.mockRestore();
+    });
+
+    it("should default to dmy if neither is found", () => {
+      const mockFormatToParts = vi.fn().mockReturnValue([
+        { type: "year", value: "2000" },
+      ]);
+
+      const mockDateTimeFormat = vi.spyOn(Intl, "DateTimeFormat").mockImplementation(() => ({
+        formatToParts: mockFormatToParts,
+      }) as any);
+
+      expect(detectDateOrder()).toBe("dmy");
+      mockDateTimeFormat.mockRestore();
+    });
+  });
+
+  describe("resolveZone", () => {
+    it("should resolve 'local' to system timezone", () => {
+      const mockResolvedOptions = vi.fn().mockReturnValue({ timeZone: "America/Los_Angeles" });
+      const mockDateTimeFormat = vi.spyOn(Intl, "DateTimeFormat").mockImplementation(() => ({
+        resolvedOptions: mockResolvedOptions,
+      }) as any);
+
+      expect(resolveZone(["local"])).toBe("America/Los_Angeles");
+      mockDateTimeFormat.mockRestore();
+    });
+
+    it("should resolve known abbreviations", () => {
+      expect(resolveZone(["utc"])).toBe("UTC");
+      expect(resolveZone(["est"])).toBe("America/New_York");
+      expect(resolveZone(["pst"])).toBe("America/Los_Angeles");
+      expect(resolveZone(["jst"])).toBe("Asia/Tokyo");
+    });
+
+    it("should resolve known cities", () => {
+      expect(resolveZone(["new", "york"])).toBe("America/New_York");
+      expect(resolveZone(["london"])).toBe("Europe/London");
+      expect(resolveZone(["tokyo"])).toBe("Asia/Tokyo");
+      expect(resolveZone(["buenos", "aires"])).toBe("America/Argentina/Buenos_Aires");
+    });
+
+    it("should resolve russian cities", () => {
+      expect(resolveZone(["нью", "йорк"])).toBe("America/New_York");
+      expect(resolveZone(["москва"])).toBe("Europe/Moscow");
+    });
+
+    it("should ignore punctuation and case", () => {
+      expect(resolveZone(["New-York"])).toBe("America/New_York");
+      expect(resolveZone(["LONdoN."])).toBe("Europe/London");
+    });
+
+    it("should resolve raw IANA timezone", () => {
+      expect(resolveZone(["Europe", "Berlin"])).toBe("Europe/Berlin");
+      expect(resolveZone(["America", "Argentina", "Mendoza"])).toBe("America/Argentina/Mendoza");
+    });
+
+    it("should return null for unknown timezones", () => {
+      expect(resolveZone(["Unknown", "City"])).toBeNull();
+      expect(resolveZone(["Invalid/Timezone"])).toBeNull();
+    });
+  });
+
+  describe("startOfToday", () => {
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+
+    it("should return midnight of the current day", () => {
+      vi.useFakeTimers();
+      vi.setSystemTime(new Date("2024-05-15T14:30:00Z"));
+
+      const expected = new Date("2024-05-15T14:30:00Z");
+      expected.setHours(0, 0, 0, 0);
+
+      expect(startOfToday()).toBe(expected.getTime());
+    });
+  });
+
+  describe("addToDate", () => {
+    it("should add days", () => {
+      const ms = new Date("2024-01-01T12:00:00Z").getTime();
+      const res = addToDate(ms, 5, "day");
+      expect(new Date(res).toISOString()).toBe("2024-01-06T12:00:00.000Z");
+    });
+
+    it("should add weeks", () => {
+      const ms = new Date("2024-01-01T12:00:00Z").getTime();
+      const res = addToDate(ms, 2, "week");
+      expect(new Date(res).toISOString()).toBe("2024-01-15T12:00:00.000Z");
+    });
+
+    it("should add months", () => {
+      const ms = new Date("2024-01-15T12:00:00Z").getTime();
+      const res = addToDate(ms, 2, "month");
+      expect(new Date(res).toISOString()).toBe("2024-03-15T12:00:00.000Z");
+    });
+
+    it("should clamp end of month correctly", () => {
+      const ms = new Date("2024-01-31T12:00:00Z").getTime();
+      const res = addToDate(ms, 1, "month");
+      // 2024 is a leap year, so Feb 29
+      expect(new Date(res).toISOString()).toBe("2024-02-29T12:00:00.000Z");
+
+      const ms2 = new Date("2023-01-31T12:00:00Z").getTime();
+      const res2 = addToDate(ms2, 1, "month");
+      // 2023 is not a leap year, so Feb 28
+      expect(new Date(res2).toISOString()).toBe("2023-02-28T12:00:00.000Z");
+    });
+
+    it("should add years", () => {
+      const ms = new Date("2024-01-15T12:00:00Z").getTime();
+      const res = addToDate(ms, 3, "year");
+      expect(new Date(res).toISOString()).toBe("2027-01-15T12:00:00.000Z");
+    });
+
+    it("should clamp end of month when adding years to leap day", () => {
+      const ms = new Date("2024-02-29T12:00:00Z").getTime();
+      const res = addToDate(ms, 1, "year");
+      expect(new Date(res).toISOString()).toBe("2025-02-28T12:00:00.000Z");
+    });
+
+    it("should handle unit ids with prefixes", () => {
+      const ms = new Date("2024-01-01T12:00:00Z").getTime();
+      const res = addToDate(ms, 5, "unit:day");
+      expect(new Date(res).toISOString()).toBe("2024-01-06T12:00:00.000Z");
+    });
+
+    it("should return original ms if amount is not an integer", () => {
+      const ms = new Date("2024-01-01T12:00:00Z").getTime();
+      const res = addToDate(ms, 1.5, "day");
+      expect(res).toBe(ms);
+    });
+
+    it("should return original ms for unhandled units", () => {
+      const ms = new Date("2024-01-01T12:00:00Z").getTime();
+      const res = addToDate(ms, 1, "hour");
+      expect(res).toBe(ms);
+    });
+  });
+
+  describe("isCalendarUnit", () => {
+    it("should return true for valid calendar units with integer amounts", () => {
+      expect(isCalendarUnit("day", 1)).toBe(true);
+      expect(isCalendarUnit("week", 5)).toBe(true);
+      expect(isCalendarUnit("month", -2)).toBe(true);
+      expect(isCalendarUnit("year", 10)).toBe(true);
+      expect(isCalendarUnit("unit:day", 1)).toBe(true);
+    });
+
+    it("should return false for floating point amounts", () => {
+      expect(isCalendarUnit("day", 1.5)).toBe(false);
+      expect(isCalendarUnit("month", 0.5)).toBe(false);
+    });
+
+    it("should return false for non-calendar units", () => {
+      expect(isCalendarUnit("hour", 1)).toBe(false);
+      expect(isCalendarUnit("minute", 1)).toBe(false);
+      expect(isCalendarUnit("unit:meter", 1)).toBe(false);
+    });
+  });
+});