Save new date post

Hi everybody,
I would like to do something simple: change the publication date of a post to a date in the future.
I added a date field in my form with the default value: {embed_post: post_date}.
When I publish the post, via the form, I recover the value of the date of the post. But if I change the value of this field for a new date in the form, it is not saved.
I do know how to change the publication date of the post with the value entered in the field of my form.
In the processors tab of the form, there is no field allowing to pass the value, so I suppose that it is done via a php code to put in the functions.php file but I don’t know how to do it.
Thank you in advance for your help