feat: infer default time format from navigator.language

app/client/lib/__tests__/datetime.test.ts

@@ -8,6 +8,7 @@ import {
 	formatShortDateTime,
 	formatTime,
 	formatTimeAgo,
+	inferDateTimePreferences,
 } from "../datetime";
 
 afterEach(() => {
@@ -89,6 +90,14 @@ describe("datetime formatters", () => {
 		expect(formatTime(sampleDate, { locale: "en-US", timeZone: "UTC", timeFormat })).toBe(expected);
 	});
 
+	test.each([
+		["en-US", { dateFormat: "MM/DD/YYYY", timeFormat: "12h" }],
+		["en-GB", { dateFormat: "DD/MM/YYYY", timeFormat: "24h" }],
+		["ja-JP", { dateFormat: "YYYY/MM/DD", timeFormat: "24h" }],
+	] as const)("infers date and time preferences for %s", (locale, expected) => {
+		expect(inferDateTimePreferences(locale)).toEqual(expected);
+	});
+
 	test("formats combined values with custom date and time preferences", () => {
 		expect(
 			formatDateTime(sampleDate, {

app/client/lib/datetime.ts

@@ -12,6 +12,8 @@ export const TIME_FORMATS = ["12h", "24h"] as const;
 export type TimeFormatPreference = (typeof TIME_FORMATS)[number];
 export const DEFAULT_TIME_FORMAT: TimeFormatPreference = "12h";
 
+const BROWSER_PREFERENCE_SAMPLE_DATE = new Date(Date.UTC(2006, 0, 2, 15, 4, 5));
+
 const DATE_PART_ORDERS = {
 	"MM/DD/YYYY": ["month", "day", "year"],
 	"DD/MM/YYYY": ["day", "month", "year"],
@@ -109,6 +111,43 @@ function formatConfiguredTime(date: Date, options: DateFormatOptions) {
 	}).format(date);
 }
 
+export function inferDateTimePreferences(locale?: string) {
+	const dateOrder = getDateTimeFormat(locale, undefined, {
+		month: "numeric",
+		day: "numeric",
+		year: "numeric",
+	})
+		.formatToParts(BROWSER_PREFERENCE_SAMPLE_DATE)
+		.flatMap((part) => {
+			if (part.type === "year" || part.type === "month" || part.type === "day") {
+				return [part.type];
+			}
+
+			return [];
+		})
+		.join("/");
+
+	let dateFormat = DEFAULT_DATE_FORMAT;
+
+	if (dateOrder === "day/month/year") {
+		dateFormat = "DD/MM/YYYY";
+	} else if (dateOrder === "year/month/day") {
+		dateFormat = "YYYY/MM/DD";
+	}
+
+	let timeFormat: TimeFormatPreference = "12h";
+	const hour12 = getDateTimeFormat(locale, undefined, { hour: "numeric" }).resolvedOptions().hour12;
+
+	if (hour12 === false) {
+		timeFormat = "24h";
+	}
+
+	return {
+		dateFormat,
+		timeFormat,
+	};
+}
+
 // 1/10/2026, 2:30 PM
 export function formatDateTime(date: DateInput, options: DateFormatOptions = {}): string {
 	return formatValidDate(

app/client/modules/auth/routes/onboarding.tsx

@@ -20,7 +20,7 @@ import { useState } from "react";
 import { useNavigate } from "@tanstack/react-router";
 import { normalizeUsername } from "~/lib/username";
 import { z } from "zod";
-import { DEFAULT_DATE_FORMAT, DEFAULT_TIME_FORMAT } from "~/client/lib/datetime";
+import { inferDateTimePreferences } from "~/client/lib/datetime";
 
 export const clientMiddleware = [authMiddleware];
 
@@ -59,10 +59,12 @@ export function OnboardingPage() {
 			return;
 		}
 
+		const { dateFormat, timeFormat } = inferDateTimePreferences(navigator.language);
+
 		const { data, error } = await authClient.signUp.email({
 			username: normalizeUsername(values.username),
-			dateFormat: DEFAULT_DATE_FORMAT,
+			dateFormat,
-			timeFormat: DEFAULT_TIME_FORMAT,
+			timeFormat,
 			password: values.password,
 			email: values.email.toLowerCase().trim(),
 			name: values.username,