clock-sync-with-cloudflare-trace.js

function syncTime() {
	const startTime = performance.now();

	const xhr = new XMLHttpRequest();
	xhr.open("GET", "/cdn-cgi/trace", true);
	xhr.onreadystatechange = () => {
		if (xhr.readyState === 4) {
			const endTime = performance.now();
			const clientTimestamp = Date.now();

			// Get the date from the cloudflare response
			const match = xhr.responseText.match(/^ts=([0-9\.]+)/m);
			if (!match) {
				console.error("Unable to get remote time");
				return;
			}

			const serverTimestamp = Number.parseFloat(match[1]) * 1000;

			console.log("Client time:", clientTimestamp);
			console.log("Server time:", serverTimestamp);

			// Find the resource timing entry for our request
			const entry = performance
				.getEntriesByType("resource")
				.find((entry) => entry.name.endsWith("/cdn-cgi/trace") && entry.startTime >= startTime);

			// Calculate time offset
			if (entry) {
				// When we received the response headers is when the server timestamp was generated
				// So we need to adjust the current time by the amount of time that has passed since then
				const timeSinceResponse = endTime - entry.responseStart;

				// Client time when response headers were received
				const clientTimeAtResponse = clientTimestamp - timeSinceResponse;

				// Offset is difference between server time and client time at that moment
				g_TimeOffset = Math.floor(serverTimestamp - clientTimeAtResponse);

				console.log("Request time:", entry.responseStart - entry.requestStart);
				console.log("Time to response start:", entry.responseStart - entry.startTime);
				console.log("Time for response:", timeSinceResponse);
			} else {
				const roundTripTime = endTime - startTime;

				// Estimate that the server timestamp was captured halfway through the request
				// This is more accurate than assuming fixed travel times in both directions
				const estimatedCurrentServerTime = serverTimestamp + roundTripTime / 2;

				g_TimeOffset = Math.floor(estimatedCurrentServerTime - clientTimestamp);
			}

			console.log("Offset:", g_TimeOffset);
			console.log("Corrected time:", clientTimestamp + g_TimeOffset);
			console.log("Sanity check:", clientTimestamp + g_TimeOffset - serverTimestamp);
		}
	};

	xhr.send();
}