My custom get_the_excerpt() can't get excerpt by ID - php

Question

On my single.php page I am trying to create a function to give me a custom length excerpt of a specific post by ID.
Below are my two functions I am running.
/* Custom get_the_excerpt to allow getting Post excerpt by ID */
function custom_get_the_excerpt($post_id) {
global $post;
$save_post = $post;
$post = get_post($post_id);
$output = get_the_excerpt($post);
$post = $save_post;
return $output;
}
/* Change Excerpt length */
function excerpt($num, $post_id = '') {
$limit = $num+1;
$excerpt = explode(' ', custom_get_the_excerpt($post_id), $limit);
array_pop($excerpt);
$excerpt = implode(" ",$excerpt)."&#8230";
echo $excerpt;
}
What I'm using to call the function.
<?php $previous = get_previous_post();
echo excerpt('30', $previous -> ID); ?>
The issue I am running into is $post is giving me the previous post information however when I pass that into get_the_excerpt it returns the current post excerpt rather than the previous post excerpt.
EDIT
Changed function to this after several people told me I can just pass the $post_id to get_the_excerpt()
/* Change Excerpt length */
function excerpt($num, $post_id = '') {
$limit = $num+1;
$excerpt = explode(' ', get_the_excerpt($post_id), $limit);
array_pop($excerpt);
$excerpt = implode(" ",$excerpt)."&#8230";
echo $excerpt;
}
Still no change.

Answer 1

Adding in setup_postdata($post); fixed my issue.
function custom_get_the_excerpt($post_id) {
global $post;
$save_post = $post;
$post = get_post($post_id);
setup_postdata($post);
$output = get_the_excerpt($post);
wp_reset_postdata();
$post = $save_post;
return $output;
}

Answer 2

the following function get two parmeters:
- $num : the number of character you would display
- $post_id: the ID of the post you want to get the content
and:
- if in the content there is the mark, return the text until it
- if $num is = 0 return the content until the first full stop
otherwise return a number of character specified on $num adding "..." at the end of the string
function my_custom_excerpt($num = 0,$post_id=''){
$post=get_post($post_id);
$content=$post->post_content;
if(strpos($content,'<!–more–>')>0){
return substr($content,0,strpos($content,'<!–more–>'));
}else{
if($num===0){
return substr($content,0,strpos($content,'.')+1);
}else{
return substr($content,0,$num).((strlen($content)>$num)?"...":"");
}
}
}