Use external form, can't read to fields type by date

1 
FormInfo startFormModel = runtimeService.getStartFormModel(procDefId, procInstId);
SimpleFormModel formModel = (SimpleFormModel) startFormModel.getFormModel();
List<FormField> fields = formModel.getFields();

when i use up code to get my form valus, one of them format is date, i use java localdatetime write to my form , but when read it is null.
help me, thanks