A Drupal 7 custom module that helps building dynamic select boxes.

common.module

<?php

/**
 *
 * Implements form_alter();
 * @param type $form
 * @param type $form_state
 * @param type $form_id
 */
function common_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'article_node_form') {

    // Add Ajax callback to parent field.
    $form['field_data_type'][LANGUAGE_NONE]['#ajax'] = array(
        'callback' => 'common_data_type_callback',
        'wrapper' => 'sub-data-container',
    );

    // Add Wrapper div to child field
    $form['field_sub_data']['#prefix'] = '<div id="sub-data-container">';
    $form['field_sub_data']['#suffix'] = '</div>';

    // When ajax callback comes or form reloads with data in it.
    // Child field should have appropriate options in it.
    if (isset($form_state['values']['field_data_type'][LANGUAGE_NONE][0]['value'])) {
      $subdata_options = array();
      switch ($form_state['values']['field_data_type'][LANGUAGE_NONE][0]['value']) {
        case 'month':
          $subdata_options = array(
              '' => '-- Please Select Month -- ',
              'jan' => 'January',
              'feb' => 'February',
              'mar' => 'Marach',
          );
          break;
        case 'week':
          $subdata_options = array(
              '' => '-- Please Select Day --',
              'sun' => 'Sunday',
              'mon' => 'Monday',
              'tue' => 'Tuesday',
          );
          break;
      }
      $form['field_sub_data'][LANGUAGE_NONE]['#options'] = $subdata_options;
    }
  }
}

/**
 * Ajax callback function on change of field_data_type
 * This function just returns the replaceable content. all logic written on form_alter.
 * @param type $form
 * @param type $form_state
 * @return type
 */
function common_data_type_callback($form, $form_state) {
  return $form['field_sub_data'];
}