Hi,
By default, we show the modified date in article page so you can open this file:
root/templates/ja_simpli/html/layouts/joomla/content/info_block/modify_date.php
and add this line of code:
<?php echo JText::sprintf(‘COM_CONTENT_PUBLISHED_DATE_ON’, ”); ?>
This language text: COM_CONTENT_PUBLISHED_DATE_ON is declared in language file of com_content: root/language/en-GB/en-GB.com_content.ini
For published / created date, you can edit the same in associated php file in this directory: root/templates/ja_simpli/html/layouts/joomla/content/info_block/
Hope this helps.