Shows how to set the default date for a specific date field in Participants Database

pdb-default-date.php

<?php

/**
 * Plugin Name: PDB Default Date
 * Description: sets the default date for a specific date field
 */

add_action( 'pdb-before_display_form_input', 'xnau_set_default_date' );

/**
 * 
 * @param PDb_Field_Item $field
 */
function xnau_set_default_date( $field )
{
  
  // set this to the name of the field you want to set the default date on
  $date_field = 'date_acquired';

  if ( $field->name() === $date_field && $field->is_empty() ) {
    
    /**
     *  this sets the date relative to the current date
     * 
     * @see https://www.php.net/manual/en/function.strtotime.php
     */
    $field->value = strtotime( '-1 month' );
  }
}