@chasity.halvorson

I apologize for the incorrect information provided earlier. Unfortunately, there is no native option in Joomla to customize the "Last-Modified" header. The "Add Last-Modified Header" option in the Global Configuration settings only enables or disables the inclusion of the "Last-Modified" header based on the enabled plugins.

To customize the "Last-Modified" header in Joomla, you will need to use a third-party extension or develop a custom Joomla plugin. Here are the general steps to create a custom plugin for modifying the "Last-Modified" header:

Create a new folder for your custom plugin in the "plugins" directory of your Joomla installation. For example, you can name it "customplugin".
Inside the "customplugin" folder, create another folder named "customplugin".
Create a PHP file named "customplugin.php" inside the second "customplugin" folder.
Open the "customplugin.php" file in a text editor and add the following code:

<?php
defined('_JEXEC') or die;

use JoomlaCMSFactory;
use JoomlaCMSPluginCMSPlugin;
use JoomlaCMSResponseResponse;

class PlgCustomplugin extends CMSPlugin
{
    public function onAfterInitialise()
    {
        $app = Factory::getApplication();

        // Check if the current request is a content item view
        if ($app->isClient('site') && $app->input->getCmd('option') === 'com_content' && $app->input->getCmd('view') === 'article')
        {
            // Modify the Last-Modified header here
            $lastModified = 'Your Custom Last-Modified Date';
            $app->getResponse()->setHeader('Last-Modified', $lastModified, true);
        }
    }
}

Save the changes to the "customplugin.php" file.
Now, you can install the custom plugin by going to the Joomla administrator panel, selecting the "Extensions" menu, and clicking on "Manage" > "Install". Choose the "Upload Package File" tab, browse and select the ZIP file containing your custom plugin, and click on "Upload & Install".
After successful installation, go to the "Extensions" menu, and click on "Plugins". Find your custom plugin in the list and enable it.

Note: Make sure to replace the placeholder "Your Custom Last-Modified Date" with the desired value for the "Last-Modified" header.

With these steps, you can create a custom Joomla plugin to modify the "Last-Modified" header based on your requirements. Remember to adjust the code based on your specific needs and validate it before using it on a live site.

#modified