# Type of request: This is a bug report

# Detailed description
S3 list buckets returns a different format for CreationDate than the one expected by the Java SDK and parsing fails. Using Java SDK 2.13.39. Also verified w/ .40, .38, .30 and .20. Will check previous Docker releases for localstack.

``Unable to unmarshall response (Text '2020-06-17T15:26:46.032838+00:00' could not be parsed at index 19). Response Code: 200, Response Text: 
software.amazon.awssdk.core.exception.SdkClientException: Unable to unmarshall response (Text '2020-06-17T15:26:46.032838+00:00' could not be parsed at index 19). Response Code: 200, Response Text: <truncated...>
``

## Expected behavior
Return CreationDate in same format as the actual S3 API call.

## Actual behavior
Returns CreationDate in slightly different format from the actual S3 API call. Fractional seconds are included.

# Steps to reproduce
Run "aws s3api list-buckets" vs "aws s3api list-buckets --endpoint-url http://localstack:4566" and note different date format for CreationDate.

## Command used to start LocalStack
docker-compose:
```
  localstack:
    image: localstack/localstack
    ports:
      - '4566:4566'
    environment:
      SERVICES: s3
      DEBUG: 1
```
## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)
See steps to reproduce.
if response.headers.get('accept-ranges') is None:
        response.headers['accept-ranges'] = 'bytes'
def is_object_expired(path):
    object_expiry = get_object_expiry(path)
    if not object_expiry:
        return False
    if dateutil.parser.parse(object_expiry) > \
            datetime.datetime.now(timezone(dateutil.parser.parse(object_expiry).tzname())):
        return False
    return True
def set_object_expiry(path, headers):
    OBJECT_EXPIRY[path] = headers.get('expires')
def get_object_expiry(path):
    return OBJECT_EXPIRY.get(path)
def is_url_already_expired(expiry_timestamp):
    if int(expiry_timestamp) < int(now_utc()):
        return True
    return False
def add_reponse_metadata_headers(response):
    if response.headers.get('content-language') is None:
        response.headers['content-language'] = 'en-US'
    if response.headers.get('cache-control') is None:
        response.headers['cache-control'] = 'no-cache'
    if response.headers.get('content-encoding') is None:
        if not uses_chunked_encoding(response):
            response.headers['content-encoding'] = 'identity'
def append_last_modified_headers(response, content=None):