WordPressで相対日付を表示する方法（2つ）

/**
 * Converts a timestamp into a human-readable relative date string.
 *
 * @param int $timestamp The timestamp to convert
 * @return string The human-readable relative date string
 */
function human_readable_relative_date( $timestamp ) {
    $time_difference = time() - $timest  // Calculate the time difference between now and the timestamp
    $seconds_in_a_day = 86400;  // Number of seconds in a day


    if ( $time_difference < 0 ) {
        return 'Date is in the future';  // Handle future dates
    } elseif ( $time_difference < $seconds_in_a_day ) {
        return 'Today at ' . date( 'H:i', $timestamp );  // Handle same-day dates
    } elseif ( $time_difference < 2 * $seconds_in_a_day ) {
        return 'Yesterday at ' . date( 'H:i', $timestamp );  // Handle one-day-old dates
    } elseif ( $time_difference < 7 * $seconds_in_a_day ) {
        $days = floor( $time_difference / $seconds_in_a_day );  // Calculate full days ago
        return $days . ($days == 1 ? ' day' : ' days') . ' ago at ' . date( 'H:i', $timestamp );  // Handle dates within the last week
    } elseif ( $time_difference < 30 * $seconds_in_a_day ) {
        $weeks = floor( $time_difference / ( 7 * $seconds_in_a_day ) );  // Calculate full weeks ago
        return $weeks . ($weeks == 1 ? ' week' : ' weeks') . ' ago at ' . date( 'H:i', $timestamp );  // Handle dates within the last month
    } elseif ( $time_difference < 365 * $seconds_in_a_day ) {
        $months = floor( $time_difference / ( 30 * $seconds_in_a_day ) );  // Calculate full months ago
        return $months . ($months == 1 ? ' month' : ' months') . ' ago at ' . date( 'H:i', $timestamp );  // Handle dates within the last year
    } else {
        $years = floor( $time_difference / ( 365 * $seconds_in_a_day ) );  // Calculate full years ago
        return $years . ($years == 1 ? ' year' : ' years') . ' ago at ' . date( 'H:i', $timestamp );  // Handle dates older than a year
    }
}


/**
 * Shortcode function to convert a given timestamp or date to a human-readable relative date string.
 *
 * @param array $atts Shortcode attributes
 * @return string The human-readable relative date string
 */
function relative_date_shortcode( $atts ) {
    $attributes = shortcode_atts(
        array(
            'timestamp' => '',  // Default value for 'timestamp' attribute
            'date' => '',       // Default value for 'date' attribute
        ),
        $atts
    );


    // If no timestamp or date attribute provided, use the post's published date
    if ( empty($attributes['timestamp']) && empty($attributes['date']) ) {
        global $post;
        if ( !is_null($post) ) {
            $attributes['timestamp'] = get_the_time('U', $post->ID);  // Get the post's timestamp
        }
    } elseif ( !empty($attributes['date']) ) {
        $attributes['timestamp'] = strtotime($attributes['date']);  // Convert 'date' attribute to timestamp
    }
    
    // Validate the timestamp
    if ( !empty($attributes['timestamp']) && is_numeric($attributes['timestamp']) ) {
        return human_readable_relative_date( $attributes['timestamp'] );  // Return the human-readable relative date
    } else {
        return 'Invalid timestamp or date';  // Return an error message if the timestamp is invalid
    }
}
add_shortcode( 'relative_date', 'relative_date_shortcode' );  // Register the shortcode

[relative_date]

if ( ! function_exists( 'twentysixteen_entry_date' ) ) :
	/**
	 * Prints HTML with date information for current post.
	 *
	 * Create your own twentysixteen_entry_date() function to override in a child theme.
	 *
	 * @since Twenty Sixteen 1.0
	 */
	function twentysixteen_entry_date() {
		$time_string = '<time class="entry-date published updated" datetime="%1$s">%2$s</time>';

		if ( get_the_time( 'U' ) !== get_the_modified_time( 'U' ) ) {
			$time_string = '<time class="entry-date published" datetime="%1$s">%2$s</time><time class="updated" datetime="%3$s">%4$s</time>';
		}

		$time_string = sprintf(
			$time_string,
			esc_attr( get_the_date( 'c' ) ),
			get_the_date(),
			esc_attr( get_the_modified_date( 'c' ) ),
			get_the_modified_date()
		);

		printf(
			'<span class="posted-on"><span class="screen-reader-text">%1$s </span><a href="%2$s" rel="bookmark">%3$s</a></span>',
			/* translators: Hidden accessibility text. */
			_x( 'Posted on', 'Used before publish date.', 'twentysixteen' ),
			esc_url( get_permalink() ),
			$time_string
		);
	}
endif;

if ( ! function_exists( 'twentysixteen_entry_date' ) ) :
	/**
	 * Prints HTML with date information for current post.
	 *
	 * Create your own twentysixteen_entry_date() function to override in a child theme.
	 *
	 * @since Twenty Sixteen 1.0
	 */
	function twentysixteen_entry_date() {
		// Get the post's published timestamp
		$published_timestamp = get_the_time('U');


		// Generate the human-readable relative date
		$relative_date = human_readable_relative_date( $published_timestamp );


		// Output the relative date
		printf(
			'<span class="posted-on"><span class="screen-reader-text">%1$s </span><a href="%2$s" rel="bookmark">%3$s</a></span>',
			/* translators: Hidden accessibility text. */
			_x( 'Posted on', 'Used before publish date.', 'twentysixteen' ),
			esc_url( get_permalink() ),
			$relative_date
		);
	}
endif;