default value when adding new item of date field
Try something like this:

//this hook or general hook_field_widget_form_alter
function hook_field_widget_date_popup_form_alter(&amp;$element, $form_state, &amp;$context){

  //only when field is specific name and during node creation form
  if ($element['#field_name'] == 'field_date' &amp;&amp; !isset($element['#entity']-&gt;nid)) {

    //if default value is not set, only for second and later elements, when first item value is set
    if (!is_array($element['#default_value']) &amp;&amp; $element['#delta'] &gt; 0 &amp;&amp; isset($form_state['values']['field_date']['und'][$element['#delta']-1]['value'])) {

      //set value
      $element['#default_value'] = array(
        //advance by 24 hours
        'value' =&gt; $form_state['values']['field_date']['und'][$element['#delta']-1]['value'] + 24 * 3600,
        //same timezone
        'timezone' =&gt; $form_state['values']['field_date']['und'][$element['#delta']-1]['timezone'],
        'timezone_db' =&gt; 'UTC',
        'date_type' =&gt; 'datestamp',
      );

      //if second value (date to) is set (by comparing value and value2)
      if ($form_state['values']['field_date']['und'][$element['#delta']-1]['value2'] != $form_state['values']['field_date']['und'][$element['#delta']-1]['value'])
        //set and advance second value by 24 hours also
        $element['#default_value']['value2'] = $form_state['values']['field_date']['und'][$element['#delta']-1]['value2'] + 24 * 3600;

      //#date_items needs to be same as #default_value
      $element['#date_items'] = $element['#default_value'];
    }
  }
}