Scheduling a category change, based on an Advanced Custom Field value, when a post is saved using a WordPress CRON job

The problem:

We had a client’s website that had a custom post type called ‘Events’. We also had a category called ‘Past Events’. This category hid the Event from the Current Events page. When an event had passed our client had to go into the backend and put the Event into the ‘Past Events’ category. As the volume of Events increased this process became too time intensive.

We wanted to maintain the mechanism of using a category to hide them as we had used this in many loops throughout the site but we needed a way to make a past event automatically add to that category at a certain date.

In addition to this we needed the date to be set via a start date or optional end date field created with the brilliant Advanced Custom Fields Plugin.

So, in a nutshell we needed to change an individual post’s category on a given date (via an ACF field) sometime in the future.

The solution:

Overview

1. Create a function that sets a WP Cron job to fire the day after an event ends
2. Create or update that Cron job every time the post is saved (ie. created or edited)
3. Create a function that puts the post into the past events category
4. Run that function when the WP Cron job is fire

The code

The code solutions here are very well commented but I have not gone into detail about why I did certain things certain ways due to time. If there is interest in me doing this then please ask in the comments and I will find the time.

Create a function that sets a WP Cron job to fire the day after an event ends.

// Create a cron job to run the day after an event happens or ends
function set_expiry_date( $post_id ) {

  // See if an event_end_date or event_date has been entered and if not then end the function
  if( get_post_meta( $post_id, $key = 'event_end_date', $single = true ) ) {

    // Get the end date of the event in unix grenwich mean time
    $acf_end_date = get_post_meta( $post_id, $key = 'event_end_date', $single = true );

  } elseif ( get_post_meta( $post_id, $key = 'event_date', $single = true ) ) {

    // Get the start date of the event in unix grenwich mean time
    $acf_end_date = get_post_meta( $post_id, $key = 'event_date', $single = true );

  } else {

    // No start or end date. Lets delete any CRON jobs related to this post and end the function.
    wp_clear_scheduled_hook( 'make_past_event', array( $post_id ) );
    return;

  }

  // Convert our date to the correct format
  $unix_acf_end_date = strtotime( $acf_end_date );
  $gmt_end_date = gmdate( 'Ymd', $unix_acf_end_date );
  $unix_gmt_end_date = strtotime( $gmt_end_date );

  // Get the number of seconds in a day
  $delay = 24 * 60 * 60; //24 hours * 60 minutes * 60 seconds

  // Add 1 day to the end date to get the day after the event
  $day_after_event = $unix_gmt_end_date + $delay;

  // Temporarily remove from 'Past Event' category
  wp_remove_object_terms( $post_id, 'past-events', 'category' );

  // If a CRON job exists with this post_id them remove it
  wp_clear_scheduled_hook( 'make_past_event', array( $post_id ) );
  // Add the new CRON job to run the day after the event with the post_id as an argument
  wp_schedule_single_event( $day_after_event , 'make_past_event', array( $post_id ) );

}

Create or update that Cron job every time the post is saved (ie. created or edited)

// Hook into the save_post_{post-type} function to create/update the cron job everytime an event is saved.
add_action( 'acf/save_post', 'set_expiry_date', 20 );

Create a function that puts the post into the past events category

// Create a function that adds the post to the past-events category
function set_past_event_category( $post_id ){

  // Set the post category to 'Past Event'
  wp_set_post_categories( $post_id, array( 53 ), true );

}

Run that function when the WP Cron job is fire

// Hook into the make_past_event CRON job so that the set_past_event_category function runs when the CRON job is fired.
add_action( 'make_past_event', 'set_past_event_category' );

Resources

Good WP Cron job tutorial
http://www.smashingmagazine.com/2013/10/schedule-events-using-wordpress-cron/

ACF save_post function
http://www.advancedcustomfields.com/resources/acfsave_post/

A plugin used for debugging the CRON jobs
https://en-gb.wordpress.org/plugins/wp-crontrol/