If you require custom code ready to go, or further help on custom coding, I would suggest contacting a https://toolset.com/contractors/ because Toolset Support can assist custom coding only limitedly.
1. As far I see, the code does not add a day to the date of the custom field, currently.
==> Did you try to print_r or var_dump() the variables you populate in your code? This is the first step in creating such code, as it helps you to understand what the data looks like and how to handle it.
2. Another thing to note is, Toolset Fields are saved with a wpcf-prefix.
Your current get_post_meta does not make usage of that, it should be:
$event_start = get_post_meta($post_id, 'wpcf-departure-date', true);
==> As mentioned, this will be easily visible as soon you var_dump() or print_r() the $variables you get in the code, it will show if you have or not expected data.
3. To add a day to a date stored in a Custom Field, if that is as Toolset Date field you will originally receive a Timestamp from the get_post_meta($post_id, 'departure-date', true);
A timestamp is a number, counting seconds from a certain date: hidden link
Now, to add one day to that precise date, you can for example simply add the value in seconds for 24 hours, but smarter it is to use PHP function that does the trick:
strtotime()
hidden link
Something like strtotime('+1 day', $timestamp); should do.
4. This allows you to add a day to your original timestamp.
So now you can update this new date to the Expiration date of the post.
That can be done directly in timestamp format as the expiration date is also stored as a timestamp.
Right now you miss the addition of 24 hours or a day to your code, however, and the prefix of the field is not yet correct.
Please let me know if you can achieve the code with the additional details.