Can’t get last modified date to appear on pages

Hi folks. I’ve created a special template for online newsletters on my site. At the top of each page, I am trying to display the month the newsletter is sent out in like “July 2007”. An example is here:

https://www.utilikilts.com/?page_id=108

The code I am using to display the date is:

<h1><?php the_time(‘F Y’) ?></h1>

The problem is that the newsletter is written before it is sent out. So I need it to display the date the page was modified, not the date it was first created. I tried using the “page date” plugin from the link below, but I can’t get it to work:

https://vapourtrails.ca/wp-plugins

I also tried changing the code in my template to:

<h1><?php the_date(‘F Y’) ?></h1>

But then nothing shows up for the date.

Need to get another newsletter out in a couple of days, and I can’t figure this out. Any help is greatly appreciated.