You can't do that using most of the services, you need to find a service that has the ability to create a .gif from the given date and then you integrate it using the img tags like this:
[img]https://seanja.com/secret/countdown/gif.php?time=2017-08-01[/img]
If you want to use the same site I just used, you change the date at the end of the URL and you are good to go. The output: