Index: src/wp-includes/default-constants.php
===================================================================
--- src/wp-includes/default-constants.php	(revision 35021)
+++ src/wp-includes/default-constants.php	(working copy)
@@ -100,13 +100,48 @@
 	 *
 	 * 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_SECONDS',  30 * DAY_IN_SECONDS    );
-	define( 'YEAR_IN_SECONDS',  365 * DAY_IN_SECONDS    );
+	if ( ! defined( 'MINUTE_IN_SECONDS' ) ) {
+		define( 'MINUTE_IN_SECONDS', 60 );
+	}
+	if ( ! defined( 'HOUR_IN_SECONDS' ) ) {
+		define( 'HOUR_IN_SECONDS',   60 * MINUTE_IN_SECONDS );
+	}
+	if ( ! defined( 'DAY_IN_SECONDS' ) ) {
+		define( 'DAY_IN_SECONDS',    24 * HOUR_IN_SECONDS   );
+	}
+	if ( ! defined( 'WEEK_IN_SECONDS' ) ) {
+		define( 'WEEK_IN_SECONDS',    7 * DAY_IN_SECONDS    );
+	}
+	if ( ! defined( 'MONTH_IN_SECONDS' ) ) {
+		define( 'MONTH_IN_SECONDS',  30 * DAY_IN_SECONDS    );
+	}
+	if ( ! defined( 'YEAR_IN_SECONDS' ) ) {
+		define( 'YEAR_IN_SECONDS',  365 * DAY_IN_SECONDS    );
+	}
 	/**#@-*/
+
+	/**#@+
+	 * Constants for expressing human-readable data sizes
+	 * in their respective number of bytes.
+	 *
+	 * @since 4.4
+	 */
+	if ( ! defined( 'KB_IN_BYTES' ) ) {
+		define( 'KB_IN_BYTES', 1024 );
+	}
+	if ( ! defined( 'MB_IN_BYTES' ) ) {
+		define( 'MB_IN_BYTES', 1024 * KB_IN_BYTES );
+	}
+	if ( ! defined( 'GB_IN_BYTES' ) ) {
+		define( 'GB_IN_BYTES', 1024 * MB_IN_BYTES );
+	}
+	if ( ! defined( 'TB_IN_BYTES' ) ) {
+		define( 'TB_IN_BYTES', 1024 * GB_IN_BYTES );
+	}
+	if ( ! defined( 'PB_IN_BYTES' ) ) {
+		define( 'PB_IN_BYTES', 1024 * TB_IN_BYTES );
+	}
+	/**#@-*/
 }
 
 /**