It still happens in version 1.6 when the object is within a collection. 

We encountered this problem as well with our server, except that we expected 
the dates to be serialized by a custom adapter.  The weird thing is that 
everything was going fine in our unit tests.
We spent a couple of hours doing remote debugging and looking at the code. It 
seems at some point during serialization, our java.util.Date objects were 
handled as if they were java.sql.Date objects.
So they were serialized following the default pattern for java.sql.Date, which 
matches our unexpected output and the unexpected output in the comment above.
I think the problem appeared in gson 1.5, where support for sql dates was 
added, and is maybe related to the fact that java.sql.Date extends 
java.util.Date...
A workaround for us was to register the same adapter for java.sql.Date than for 
java.util.Date:

Gson gson = new GsonBuilder()
        .......
        .registerTypeAdapter(Date.class, new DateTypeAdapter())
        .registerTypeAdapter(java.sql.Date.class, new DateTypeAdapter())
        .......
        .create();

That woule be cool to fix that in 1.7!

This should be a higher priority defect since working with dates is very 
common. Hope this will be fixed as soon as possible.

[deleted comment]

JsonSerializationVisitor.visitFieldUsingCustomHandler(FieldAttributes, Type, 
Object) line: 195

Object obj = f.get(parent);

In f.get() method, "field.get(instance)" returns Timestamp object for 
java.util.Date class.

field is java.lang.reflect.Field


and then


ObjectTypePair.getActualTypeIfMoreSpecific(Type, Class<?>) line: 87 

if (typeAsClass.isAssignableFrom(actualClass)) {
    type = actualClass;
}

The "type" which is "java.util.Date" is assigned by "actualClass" which is 
"java.sql.Timestamp".

On startup when sets datePattern for GsonBuilder, code creates dateTypeAdapter 
for Date.class in GsonBuilder.addTypeAdaptersForDate method:


if (datePattern != null && !"".equals(datePattern.trim())) {
   dateTypeAdapter = new DefaultDateTypeAdapter(datePattern);
} 
...
if (dateTypeAdapter != null) {
  if (!serializers.hasSpecificHandlerFor(Date.class)) {
      serializers.register(Date.class, dateTypeAdapter);
  }
....

but line 530 of GsonBuilder while "serializers" is being filled with 
defaultSerializers, dateTypeAdapter for Timestamp.class is created with default 
SimpleDateFormat.

Hi again, 

Gson is trying to serialize more specific field type. When reading Date type 
from database the Date field of domain object is set to java.sql.Timestamp or 
java.sql.Date

A workaround is to register custom adapter according to your Date instance type 
read from database:

registerTypeAdapter(Timestamp.class, new DateSerializer())
or
registerTypeAdapter(java.sql.Date.class, new DateSerializer())



in source code we can change GsonBuilder.addTypeAdaptersForDate method like 
this:

....
   if (dateTypeAdapter != null) {
      if (!serializers.hasSpecificHandlerFor(Date.class)) {
          serializers.register(Date.class, dateTypeAdapter);
      }
      if (!serializers.hasSpecificHandlerFor(Timestamp.class)) {
          serializers.register(Timestamp.class, dateTypeAdapter);
      }
      if (!serializers.hasSpecificHandlerFor(java.sql.Date.class)) {
          serializers.register(java.sql.Date.class, dateTypeAdapter);
      }


      if (!deserializers.hasSpecificHandlerFor(Date.class)) {
          deserializers.register(Date.class, dateTypeAdapter);
      }
      if (!deserializers.hasSpecificHandlerFor(Timestamp.class)) {
          deserializers.register(Timestamp.class, dateTypeAdapter);
      }
      if (!deserializers.hasSpecificHandlerFor(java.sql.Date.class)) {
          deserializers.register(java.sql.Date.class, dateTypeAdapter);
      }
    }
....

This issue was closed by revision r762.