--- a/js/src/builtin/Intl.js
+++ b/js/src/builtin/Intl.js
@@ -2223,17 +2223,22 @@ function resolveDateTimeFormatInternals(
     internalProps.calendar = r.ca;
     internalProps.numberingSystem = r.nu;
 
     // Compute formatting options.
     // Step 14.
     var dataLocale = r.dataLocale;
 
     // Steps 15-17.
-    internalProps.timeZone = lazyDateTimeFormatData.timeZone;
+    var tz = lazyDateTimeFormatData.timeZone;
+    if (tz === undefined) {
+        // Step 16.
+        tz = DefaultTimeZone();
+    }
+    internalProps.timeZone = tz;
 
     // Step 18.
     var formatOpt = lazyDateTimeFormatData.formatOpt;
 
     // Steps 27-28, more or less - see comment after this function.
     var pattern = toBestICUPattern(dataLocale, formatOpt);
 
     // Step 29.
@@ -2343,19 +2348,16 @@ function InitializeDateTimeFormat(dateTi
 
         // Step 15.b.
         var timeZone = intl_IsValidTimeZoneName(tz);
         if (timeZone === null)
             ThrowRangeError(JSMSG_INVALID_TIME_ZONE, tz);
 
         // Step 15.c.
         tz = CanonicalizeTimeZoneName(timeZone);
-    } else {
-        // Step 16.
-        tz = DefaultTimeZone();
     }
     lazyDateTimeFormatData.timeZone = tz;
 
     // Step 18.
     var formatOpt = new Record();
     lazyDateTimeFormatData.formatOpt = formatOpt;
 
     // Step 19.