How do you add help text to a form item?
The description doesn't live under #attributes, but under the element directly.

So you must do something like this to get it to work:

$form['field_event_date_value2']['#description'] = 'DD/MM/YYYY';


The HTML class is not the fieldname, but rather something like field_event_date_value2 in your case.

If that is not the correct fieldname, you can try to print_r(array_keys($form)) to see if you can see what the correct name is.