With this guide I want to explain how easily you can add invoice issue date to list of orders in your shop back office. Guide is based on controller modification: AdminOrdersController that is a part of PrestaShop core. Guide can be used in PrestaShop v1.5, v1.6 and in recently released v1.7 too. Details below.

Add invoice date field to list of orders

Firstly please open this file: /controllers/admin/AdminOrdersContorller.php. This file is responsible for Orders > orders section in your shop back office. File contains code that build order detail page and also page, where you see list of orders. Search there for function __construct(); it begins with code:

public function __construct()
{
...
...
...
}

Inside this function you can find definition of "fields list" variable. This variable contains list of fields that are visible on list of orders. These fields are: ID, reference, Customer, Total, Status etc. If we want to add new field with "invoice issue date" we have to alter this variable.

Whole default code of "fields_list" variable looks like:

$this->fields_list = array(
            'id_order' => array(
                'title' => $this->l('ID'),
                'align' => 'text-center',
                'class' => 'fixed-width-xs'
            ),
            'reference' => array(
                'title' => $this->l('Reference')
            ),
            'new' => array(
                'title' => $this->l('New client'),
                'align' => 'text-center',
                'type' => 'bool',
                'tmpTableFilter' => true,
                'orderby' => false,
                'callback' => 'printNewCustomer'
            ),
            'customer' => array(
                'title' => $this->l('Customer'),
                'havingFilter' => true,
            ),
        );

        if (Configuration::get('PS_B2B_ENABLE')) {
            $this->fields_list = array_merge($this->fields_list, array(
                'company' => array(
                    'title' => $this->l('Company'),
                    'filter_key' => 'c!company'
                ),
            ));
        }

        $this->fields_list = array_merge($this->fields_list, array(
            'total_paid_tax_incl' => array(
                'title' => $this->l('Total'),
                'align' => 'text-right',
                'type' => 'price',
                'currency' => true,
                'callback' => 'setOrderCurrency',
                'badge_success' => true
            ),
            'payment' => array(
                'title' => $this->l('Payment')
            ),
            'osname' => array(
                'title' => $this->l('Status'),
                'type' => 'select',
                'color' => 'color',
                'list' => $this->statuses_array,
                'filter_key' => 'os!id_order_state',
                'filter_type' => 'int',
                'order_key' => 'osname'
            ),
            'date_add' => array(
                'title' => $this->l('Date'),
                'align' => 'text-right',
                'type' => 'datetime',
                'filter_key' => 'a!date_add'
            ),
            'id_pdf' => array(
                'title' => $this->l('PDF'),
                'align' => 'text-center',
                'callback' => 'printPDFIcons',
                'orderby' => false,
                'search' => false,
                'remove_onclick' => true
            )
        ));

add there lines that i highlighted below:

$this->fields_list = array(
            'id_order' => array(
                'title' => $this->l('ID'),
                'align' => 'text-center',
                'class' => 'fixed-width-xs'
            ),
            'reference' => array(
                'title' => $this->l('Reference')
            ),
            'new' => array(
                'title' => $this->l('New client'),
                'align' => 'text-center',
                'type' => 'bool',
                'tmpTableFilter' => true,
                'orderby' => false,
                'callback' => 'printNewCustomer'
            ),
            'customer' => array(
                'title' => $this->l('Customer'),
                'havingFilter' => true,
            ),
        );

        if (Configuration::get('PS_B2B_ENABLE')) {
            $this->fields_list = array_merge($this->fields_list, array(
                'company' => array(
                    'title' => $this->l('Company'),
                    'filter_key' => 'c!company'
                ),
            ));
        }

        $this->fields_list = array_merge($this->fields_list, array(
            'total_paid_tax_incl' => array(
                'title' => $this->l('Total'),
                'align' => 'text-right',
                'type' => 'price',
                'currency' => true,
                'callback' => 'setOrderCurrency',
                'badge_success' => true
            ),
            'payment' => array(
                'title' => $this->l('Payment')
            ),
            'osname' => array(
                'title' => $this->l('Status'),
                'type' => 'select',
                'color' => 'color',
                'list' => $this->statuses_array,
                'filter_key' => 'os!id_order_state',
                'filter_type' => 'int',
                'order_key' => 'osname'
            ),
            'date_add' => array(
                'title' => $this->l('Date'),
                'align' => 'text-right',
                'type' => 'datetime',
                'filter_key' => 'a!date_add'
            ),
            'invoice_date' => array(
                'title' => $this->l('Invoice Date'),
                'align' => 'text-right',
                'type' => 'datetime',
                'filter_key' => 'a!invoice_date'
            ),
            'id_pdf' => array(
                'title' => $this->l('PDF'),
                'align' => 'text-center',
                'callback' => 'printPDFIcons',
                'orderby' => false,
                'search' => false,
                'remove_onclick' => true
            )
        ));

After you will modify the "fields_list" variable - just save the changes. New field with "Invoice issue date" should be visible on list of orders.