[builtins] Apply ES2016 conditional default timezone semantics

src/dateparser-inl.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_DATEPARSER_INL_H_
 #define V8_DATEPARSER_INL_H_
 
 #include "src/char-predicates-inl.h"
 #include "src/dateparser.h"
 #include "src/unicode-cache-inl.h"
@@ skipping 325 matching lines @@
         if (!scanner->SkipSymbol(':')) return DateToken::Invalid();
         if (!scanner->Peek().IsFixedLengthNumber(2) ||
             !TimeComposer::IsMinute(scanner->Peek().number())) {
           return DateToken::Invalid();
         }
         tz->SetAbsoluteMinute(scanner->Next().number());
       }
     }
     if (!scanner->Peek().IsEndOfInput()) return DateToken::Invalid();
   }
-  // Successfully parsed ES5 Date Time String. Default to UTC if no TZ given.
-  if (tz->IsEmpty()) tz->Set(0);
+  // Successfully parsed ES5 Date Time String.
+  // ES#sec-date-time-string-format Date Time String Format
+  // "When the time zone offset is absent, date-only forms are interpreted
+  //  as a UTC time and date-time forms are interpreted as a local time."
+  if (tz->IsEmpty() && time->IsEmpty()) {
+    tz->Set(0);
+  }
   day->set_iso_date();
   return DateToken::EndOfInput();
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_DATEPARSER_INL_H_