Driven NEP: disabling date fields results in validation failure in node form

I need to know exactly how to reproduce it

I tried with dropdown widget and there was no trouble (date-6.x-2.4 but 2.6 doesn't look so different)

since NEP only turns #access into FALSE, it should work,
it is unobtrusive, and complies FAPI specs

PS: I might be missing something, this was just a quick trial

configuration of the date field would be helpful (for instance, you don't mention its granularity)

the only place in the whole date module where that string exists is in date_elements.inc, near line 480

    if (!$field['required'] && (empty($from_date) || empty($to_date))) {
      $item = date_element_empty($element, $form_state);
      $errors[] = t('The dates are invalid.');
    }

and it clearly says if (!$field['required']) ... which is precisely the opposite of what you describe regarding making the field required

its really odd,
I'll have to try it in a fresh install

BTW, which value did you assigned to the date field on node creation?

PS: the date I tried in my test has
- Default value: blank
- Default value for To date: same as From date
- Years back and forward: -3:+3
- ... oh.. it is not required (then my test was not accurate)
- granularity up to the minute

indeed, making it required brings the error

as I previously said @#1,
Driven NEP only turns #access into FALSE
which leaves $element['#post'] without a $field_name index (near line 391)
while the return statement (near line 385) saves the situation for non-required dates

then, turning #access=FALSE makes $element['#post'][$field_name] undefined, then $posted is also undefined (which is already a bug, but doesn't causes the current issue)
and immediately bellow it $field results undefined too, since $form_state['#field_info'][$field_name] is also undefined (right now I won't look why it is, I guess it is part of date's submission workflow)
Note: $field_name and $element['#field_name'] hold the same value in my test, that is why I used them indistinctly in the above description

  // If the whole field is empty and that's OK, stop now.
  if (empty($element['#post'][$field_name]) && !$element['#required']) {
    return;
  }
  
  // Repeating dates have a different form structure, so get the
  // right item values.
  $item = isset($form_values[$field_name]['rrule']) ? $form_values[$field_name] : $form_values[$field_name][$delta];
  $posted = isset($form_values[$field_name]['rrule']) ? $element['#post'][$field_name] : $element['#post'][$field_name][$delta];
    
  $field = $form_state['#field_info'][$element['#field_name']];

then, if $field is undefined the code @#5 triggers the error because !$field['required'] yields TRUE

according to FAPI #access=FALSE should be supported by date module,
ergo, not our bug

PS: I recommend opening a new issue on the date module, because they wouldn't distinguish their fault mixed with Driven API

BTW for the date module my proposal would be the following

  // If the whole field is empty and that's OK, stop now.
  if (empty($element['#post'][$field_name]) && (!$element['#required'] || (isset($element['#access']) && !$element['#access']))) {
    return;
  }

moved to its actual project