Hello

.gitignore

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+/node_modules
+/package-lock.json

index.js

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+#!/usr/bin/env node
+
+var { Client } = require('undici');
+var { parseArgs } = require('node:util');
+
+var { values, positionals } = parseArgs({
+	options: {
+		help: {
+			type: 'boolean'
+		}
+	},
+	allowPositionals: true,
+});
+
+var help =
+`Usage: webtimediff <URL>
+Calculate system clock difference between this host and a web server.
+
+  --help    display this help and exit
+`;
+
+if (values.help || positionals.length != 1) {
+	console.log(help);
+	return;
+}
+
+function sleep(ms) {
+	return new Promise((res, rej) => setTimeout(_ => res(), ms));
+}
+
+((async function(url) {
+	var conn = new Client(url.origin, {
+		pipelining: 1000,
+		headersTimeout: 10e3,
+		bodyTimeout: 10e3,
+	});
+
+	let step = 20;
+	let end = 2000;
+	let errors = [];
+	let resps = [];
+	let firstRemoteTime = undefined;
+	let foundChange = undefined;
+
+	await conn.request({
+		method: "HEAD",
+		path: url.pathname + url.search,
+		blocking: false,
+		idempotent: true,
+	});
+
+	for (let i = 0; (!foundChange) && i < end; i += step) {
+		if (errors.length > 0)
+			break;
+
+		let date = new Date();
+		let ii=i;
+		let res = conn.request({
+			method: "OPTIONS",
+			path: url.pathname + url.search,
+			blocking: false,
+			idempotent: true,
+		}).then(e => (
+			e.date = new Date(e.headers.date),
+			firstRemoteTime = firstRemoteTime ?? e.date,
+			e.madeAt = date,
+			e.doneAt = new Date(),
+			foundChange = foundChange ??
+				((firstRemoteTime && (firstRemoteTime.getTime() != e.date.getTime()))
+					? {
+						remoteTime: e.date.getTime()/1000,
+						beginTime: e.madeAt.getTime()/1000,
+						endTime: e.doneAt.getTime()/1000,
+					}
+					: foundChange),
+			e));
+		resps.push(res);
+
+		if (i + step < end)
+			await sleep(step);
+	}
+
+	if (errors.length > 0)
+		await Promise.all(errors);
+
+	Promise.all(resps).catch(e => {});
+
+	let firstReq = [await (resps[0])].map(res => ({
+		remoteTime: res.date.getTime()/1000,
+		beginTime: res.madeAt.getTime()/1000,
+		endTime: res.doneAt.getTime()/1000,
+	}))[0];
+
+	let reqEnds = (foundChange.endTime - firstReq.endTime);
+	let reqBegins = (foundChange.beginTime - firstReq.beginTime)
+
+	let lag = reqEnds / reqBegins;
+
+	if (!(lag < 1.08)) {
+		console.error("WARNING: Big Lag ("
+			+reqEnds+
+			"s / "
+			+reqBegins+
+			"s) detected, reported result may be inaccurate.");
+	}
+
+	if (!foundChange) {
+		throw new Error("The remote clock does not work.");
+	}
+
+	let diff = (foundChange.remoteTime - (foundChange.beginTime + foundChange.endTime) / 2);
+
+	console.log("remote_time = local_time "+(diff < 0 ? "-" : "+")+" " + Math.abs(diff)+ " s");
+	process.exit();
+})(new URL(positionals[0])))

package.json

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+{
+  "dependencies": {
+    "undici": "^7.8.0"
+  }
+}