Add PubDate for each article to RSS-feed

Hi,

I want to use the Mailchimp RSS based campaign. In order to use this RSS campaign each item of my feed must contain a pubdate tag. How do I add a pubdate to my feed?

The feed is: https://www.citymarketingonline.nl/feed/rss/

Which php file must I change in order to add the pubdate?

Kind Regards,

Jean-Pierre