Countdown plugin that will be set in WP admin on a post and will be shown in front end in the post

I want this plugin to have 3 custom fields, let's call them A and B, A top of the interval and B the bottom of the interval with C being the time that it takes for the numbers to decrease from A to B. I want to be able to set these three variables from the wp-admin part of a Wordpress site when I'm editing or creating a post. They should be in the form of three simple custom fields attached to any post. The countdown should be visible in the post. I'm guessing you will do this with a shortcode. I will also want to place the coundown code outside the content(), but still within the loop so I will also need the simple php code that I will have to insert. Another thing that I should mention is that after the initial values are set, if I happen to edit and save the post, the countdown should start from the values that are in the custom fields aka the countdown custom fields should be editable at any point.