To get the time three hours from what their computer currently thinks the time is you would use:
var time = new Date();
alert('In three hours it will be ' +time.getHours() +':'+time.getMinutes()+':'+time.getSeconds());
The code to get the time three hours from now would then be:
var time = new $D();
alert('In three hours it will be '+time.format('H:i:s'));
Of course the extra code for the library isn't worth it unless you are actually going to be doing a lot of date and time processing and want to make it easier to read like this.