add_action upon post published with custom field

Hello, I'm hoping you can help with this.

I have users submitting posts that are published directly. I do not want to administer them, so that is not an option for me.

They are submitting, among other things, a date custom field called "event_date".
I have a little function that converts the event_date into a timestamp and saves it in a new custom field called "date_value".

I have the conversion and save happening on 'save_post' like this:
add_action('save_post', 'mb_convertToTimestamp', $postid);

This works fine for posts submmitted through the Wordpress backend.
However it doesn't seem to work with posts submitted through the gravity form. Date_value is empty.

Any thoughts? I've also tried it on "publish_post" and it doesn't work either.