Install the WordPress plugin Calculator Builder

Installation option 1: Find and install this plugin in the Plugins -> Add a new section of your wp-admin
Installation option 2: Download the zip file, then upload the plugin via the wp-admin in the Plugins -> Add a new section. Or unzip the archive and upload the folder to the plugins directory /wp-content/plugins/ via FTP
Press Activate when you have installed the plugin via the dashboard or press Activate in the Plugins list
Go to the Calculator Builder (CalcHub) section that will appear in your main menu on the left
Click Add new to create your first countdown
Build calculator
Click Save
Copy and paste the shortcode, such as [Calculator id=1] to where you want the countdown to appear.
If you want it to appear everywhere on your site, you can insert it for example in your header.php, like this: <?php echo do_shortcode('[Calculator id=1]');?>