Support of dynamic banner

I would like to have in banner date&time of the build. I used the pseudocode below, but the initial value stays in the output constantly. Am I doing something wrong, or this is not supported right now?

let bannerForCSS = '/* initial value */';

const esbuildOptions = {
  banner: {
    css: bannerForCSS,
  },
  plugins: [
    {
      name: 'timestamp',
      setup(build) {
        build.onStart(() => {
          const timestamp = new Date().toLocaleTimeString('en-UK');
          bannerForCSS = `/* Automatic build at: ${timestamp} */`;
        });
      },
    }
  ]
};

const ctx = await esbuild.context(esbuildOptions);
await ctx.watch();

Start with the banner.css option and the plugin setup, build.onStart, context, and watch entry points shown in the example. Determine whether the banner is evaluated once or can change on rebuild; done means the supported behavior and any required change are clearly established.