Hi Kevin

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In regards to your question, this would be a case of editing the values using a custom script or possibly a plugin – it also depends on how they’ve been added in the first place.

Thanks

Hi Kevin

So you would create a WP loop to go through the post type you want, and in that loop you would update post meta like this:
https://wordpress.stackexchange.com/questions/245978/how-can-i-update-post-meta-inside-a-wp-query-loop-or-the-wordpress-loop

Now, what you actually want to do, is replace the original post meta with the new formatted data.

So you would get the existing post meta (using get_post_meta function), convert it using PHP date function:
https://wordpress.stackexchange.com/a/12039
And save it back to the post using update_post_meta all in that loop.

Thanks