Add “Posted on” to post date

The post date is added by twentyfourteen_posted_on as shown in content.php. This function is in /inc/template-tags.php and is correctly wrapped in an if ( ! function_exists( 'twentyfourteen_posted_on' ) ) conditional so you can declare the same function in your child theme and modify it to your heart’s content.

As such you could add a function like this to your theme’s functions.php file and that should do the trick (note the addition of “Posted on ” in the printf function:

function twentyfourteen_posted_on() {
        if ( is_sticky() && is_home() && ! is_paged() ) {
                echo '<span class="featured-post">' . __( 'Sticky', 'twentyfourteen' ) . '</span>';
        }

        // Set up and print post meta information.
        printf( 'Posted on <span class="entry-date"><a href="https://wordpress.stackexchange.com/questions/220435/%1$s" rel="bookmark"><time class="entry-date" datetime="%2$s">%3$s</time></a></span> <span class="byline"><span class="author vcard"><a class="url fn n" href="%4$s" rel="author">%5$s</a></span></span>',
                esc_url( get_permalink() ),
                esc_attr( get_the_date( 'c' ) ),
                esc_html( get_the_date() ),
                esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ),
                get_the_author()
        );
}