Index: src/wp-includes/class-wp-customize-nav-menus.php
===================================================================
--- src/wp-includes/class-wp-customize-nav-menus.php	(revision 33663)
+++ src/wp-includes/class-wp-customize-nav-menus.php	(working copy)
@@ -467,7 +467,11 @@
 		// Menu loactions.
 		$locations     = get_registered_nav_menus();
 		$num_locations = count( array_keys( $locations ) );
-		$description   = '<p>' . sprintf( _n( 'Your theme contains %s menu location. Select which menu you would like to use.', 'Your theme contains %s menu locations. Select which menu appears in each location.', $num_locations ), number_format_i18n( $num_locations ) );
+		if ( 1 == $num_locations ) {
+			$description = '<p>' . __( 'Your theme supports one menu location. Select which menu you would like to use.' );
+		} else {
+			$description = '<p>' . sprintf( _n( 'Your theme contains %s menu location. Select which menu you would like to use.', 'Your theme contains %s menu locations. Select which menu appears in each location.', $num_locations ), number_format_i18n( $num_locations ) );
+		}
 		$description  .= '</p><p>' . __( 'You can also place menus in widget areas with the Custom Menu widget.' ) . '</p>';
 
 		$this->manager->add_section( 'menu_locations', array(
Index: src/wp-includes/default-constants.php
===================================================================
--- src/wp-includes/default-constants.php	(revision 33663)
+++ src/wp-includes/default-constants.php	(working copy)
@@ -95,12 +95,20 @@
 	// Constants for features added to WP that should short-circuit their plugin implementations
 	define( 'WP_FEATURE_BETTER_PASSWORDS', true );
 
-	// Constants for expressing human-readable intervals
-	// in their respective number of seconds.
+	/**
+	 * Constants for expressing human-readable intervals
+	 * in their respective number of seconds.
+	 *
+	 * Please note that these values are approximate and are provided for convenience.
+	 * For example, MONTH_IN_SECONDS wrongly assumes every month has 30 days.
+	 *
+	 * If you need more accuracy please consider using the DateTime class (http://php.net/manual/class.datetime.php).
+	 */
 	define( 'MINUTE_IN_SECONDS', 60 );
 	define( 'HOUR_IN_SECONDS',   60 * MINUTE_IN_SECONDS );
 	define( 'DAY_IN_SECONDS',    24 * HOUR_IN_SECONDS   );
 	define( 'WEEK_IN_SECONDS',    7 * DAY_IN_SECONDS    );
+	define( 'MONTH_IN_DAYS',     30 * DAY_IN_SECONDS    );
 	define( 'YEAR_IN_SECONDS',  365 * DAY_IN_SECONDS    );
 }