diff --git a/src/engine/datetime.test.ts b/src/engine/datetime.test.ts
index c327c52..ee10c62 100644
--- a/src/engine/datetime.test.ts
+++ b/src/engine/datetime.test.ts
@@ -13,38 +13,12 @@ describe("datetime", () => {
       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 dmy (e.g. en-GB)", () => {
+      expect(detectDateOrder("en-GB")).toBe("dmy");
     });
 
-    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 detect mdy (e.g. en-US)", () => {
+      expect(detectDateOrder("en-US")).toBe("mdy");
     });
 
     it("should default to dmy if neither is found", () => {
diff --git a/src/engine/datetime.ts b/src/engine/datetime.ts
index 226a331..078d4f4 100644
--- a/src/engine/datetime.ts
+++ b/src/engine/datetime.ts
@@ -2,8 +2,8 @@
 import { DateOrder } from "./types";
 
 /** Does the OS locale write the day or the month first? (31.12 vs 12/31) */
-export function detectDateOrder(): DateOrder {
-  const parts = new Intl.DateTimeFormat(undefined, { day: "numeric", month: "numeric", year: "numeric" })
+export function detectDateOrder(locale?: string): DateOrder {
+  const parts = new Intl.DateTimeFormat(locale, { day: "numeric", month: "numeric", year: "numeric" })
     .formatToParts(new Date(2000, 11, 31));
   for (const p of parts) {
     if (p.type === "day") return "dmy";