[builtins] Apply ES2016 conditional default timezone semantics

src/dateparser-inl.h

Index: src/dateparser-inl.h
diff --git a/src/dateparser-inl.h b/src/dateparser-inl.h
index 47a7c6e7ff1e8c8f1b574ad8478574ad80be3d49..fd4bed2df6a7071a2138012ff575960baffa8ce0 100644
--- a/src/dateparser-inl.h
+++ b/src/dateparser-inl.h
@@ -343,8 +343,13 @@ DateParser::DateToken DateParser::ParseES5DateTime(
     }
     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();
 }