Displaying subtitle in recent post widget

On my travelblog (twenty seventeen theme) I really want to display the subtitle in the recent post widget in my sidebar. A bit like the date is showing now. (sorry it is in dutch)

I tried it with this in the widget

<?php $subtitle = get_the_subtitle($this->post->ID, ‘_subtitle’, true); ?>
<p class=“entry-subtitle”><?php echo $subtitle;?>

But that didn’t work because it only took the subtitle from the first blog every time.Could you please help me?

The page I need help with: [log in to see the link]