Hi @huwrowlands
I think you need to use the cron job and check if the closing date for each post has passed or not. If the date has passed, you can update the other fields by using the update_field() function. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/update_field/.
To learn how to use the cron job, please take a look at this page: https://www.smashingmagazine.com/2013/10/schedule-events-using-wordpress-cron/.
I hope this helps 🙂
