Programmatically set date fields in Drupal 7: Date, Date (ISO Format) and Date (Unix Timestamp)
Here is a code that treat site timezone right for me:

 $node-&gt;field_date[LANGUAGE_NONE][0] = array(
    'value' =&gt; format_date(strtotime('now'), 'custom', 'Y-m-d H:i:s', 'UTC'),
    'timezone' =&gt; 'UTC',  
    'timezone_db' =&gt; 'UTC',
  );