Computed values drupal 7 node presave

week_of.module

<?php

function week_of_form_alter(&$form, &$form_state, $form_id) {
  if($form_id == 'eligible_post_node_form') {
    $form['field_week_of']['#access'] = FALSE;
  }
}

function week_of_node_presave($node) {
  if($node->type == "eligible_post") {
    $wrapper = entity_metadata_wrapper('node', $node);
    $unix_time = $wrapper->field_post_date->value();
    $date = date('Y-m-d' ,$unix_time);
    $formated_date = "Week of ".date('d F Y', strtotime('monday this week', strtotime($date)));
    $wrapper->field_week_of->set($formated_date);
  }
}