public class TimestampXmlAdapter extends XmlAdapter<String, Timestamp>
{
    private static final SimpleDateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss");

    public static String format(Timestamp timestamp)
    {
        String formatted = "";

        if (timestamp != null)
        {
            formatted = dateFormat.format(timestamp);
        }

        return formatted;
    }

    @Override
    public String marshal(Timestamp timestamp) throws Exception
    {
        return TimestampXmlAdapter.format(timestamp);
    }

    @Override
    public Timestamp unmarshal(String timestamp) throws Exception
    {
        return new Timestamp(dateFormat.parse(timestamp).getTime());
    }

}

<MyObject>
    <id>111-222</id>
    <activeDate></activeDate>
</MyObject>

Local Exception Stack: 
Exception [EclipseLink-3001] (Eclipse Persistence Services - 2.0.1.v20100213-r6600): org.eclipse.persistence.exceptions.ConversionException
Exception Description: The object [], of class [class java.lang.String], could not be converted to [class java.sql.Timestamp].
	at org.eclipse.persistence.exceptions.ConversionException.couldNotBeConverted(ConversionException.java:71)

    public Timestamp unmarshal(String timestamp) throws Exception
    {
        if (timestamp == null || timestamp.equals(""))
        {
            return null;
        }
        return new Timestamp(dateFormat.parse(timestamp).getTime());
    }

Local Exception Stack: 
Exception [EclipseLink-33] (Eclipse Persistence Services - 2.0.1.v20100213-r6600): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: Trying to invoke [setActiveDate] on the object with the value [].  The number of actual and formal parameters differs, or an unwrapping conversion has failed.
Internal Exception: java.lang.IllegalArgumentException: argument type mismatch
Mapping: org.eclipse.persistence.oxm.mappings.XMLDirectMapping[activeDate-->activeDate/text()]
Descriptor: XMLDescriptor(my.package.model.MyObject --> [DatabaseTable(MyObject)])
	at org.eclipse.persistence.exceptions.DescriptorException.illegalArgumentWhileSettingValueThruMethodAccessor(DescriptorException.java:691)