App.js

import logo from "./logo.svg";
import "./App.css";

function App() {
  const ms = Date.now() - parseInt(window.location.search.substring(1), 10);
  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <h3>Loaded in {ms}ms.</h3>
        <a
          className="App-link"
          href="https://reactjs.org"
          target="_blank"
          rel="noopener noreferrer"
        >
          Learn React
        </a>
      </header>
    </div>
  );
}

export default App;

print-timestamp.zig

const std = @import("std");

pub fn main() anyerror!void {
    var stdout = std.io.getStdOut();
    var writer = stdout.writer();
    const timestamp = std.time.milliTimestamp();
    writer.print("{d}", .{timestamp}) catch unreachable;
}