Result of shortcode will appear after entire code of that shortcode is processed.

PHP can do what you are trying to do through javascript as it is just adding information to an element. As both calculations are ready through PHP processing, you can just echo full 'webinartime' element without handing over that job to javascript. From your content where you used the element or etc, you can remove from there and let the PHP code display it already filled with the calculations.

Hope I understood your situation right from your description.

Result of shortcode will appear after entire code of that shortcode is processed.

PHP can do what you are trying to do through javascript as it is just adding information to an element. As both calculations are ready through PHP processing, you can just echo full 'webinartime' element without handing over that job to javascript. From your content where you used the element div or span etc, you can remove from there and let the PHP code display it already filled with the calculations.

Hope I understood your situation right from your description.