This is an extension that creates three new columns in newsletter_subscriber table: created_at, confirmed_at and unsubscribed_at.
The purpose is to store timestamps whenever a newsletter subscriber either subscribes, confirms subscription or unsubscribes from the newsletter.
These can be seen in admin grid under Newsletter > Newsletter Subscribers.
Just copy to your Magento project root.
Developed for Magento CE 1.9.2.4, not tested on any other. Most likely works with Magento CE >= 1.9, or even older. You're welcome to submit a PR and document in case you've tested with versions prior to 1.9 CE!