Is there a way in webpack watch mode to display on screen the timestamp when webpack last updated the build?

you can add a custom plugin like:

config.plugins.push(function(){
    this.plugin('done', function(stats) {
        console.log(('\n[' + new Date().toLocaleString() + ']') + ' Begin a new compilation.\n');
    });
});

The answer of datou3600 is awesome, but why not to be better?

Adding a little delay:

1. The text is placed at the end
2. A screen blinks noticeable to an eye

Here is the code:

config.plugins.push(function(){
    this.plugin('done', function(stats) {
        setTimeout(
            () => {
                console.log(('\n[' + new Date().toLocaleString() + ']') + ' --- DONE.\n');
            },
            100
        );
    });
});

Install webpack-watch-time-plugin.

It displays the time when watcher rebuild happens.