How to update and check transients on Wordpress. More at https://xilab.co.

wp_update_transient.php

/*
* @param $tag = tag to use for this transient
* @param $element = value to associate the transient
* @param $duration / $expiration = time in seconds > 3600*24 for 1 day
* @return true > if needed to update | false > if still not expired
*/

function update_transient($tag, $element, $duration, $expiration){

		$elements_in_tag = get_transient($tag);
		$need_to_update = isset($elements_in_tag[$element]) && $elements_in_tag[$element] > (time() - ($duration));

		if($need_to_update){
			$elements_in_tag[$element] = time();
			set_transient($tag, $elements_in_tag, $expiration);
			return true;
		}

		return false;

}