Rules add date variable from webform submission
Later I realised that I need to run a custom php code, so I wrote this, which actually works:

// use the submitted time value from the webform and convert it to time, also use it for an end date one year later
$time = strtotime($data['components']['submitted time']['value'][0]);
$time2 = strtotime('+1 year', $time);

//convert to date formats to the format used in these fields
$startdate = date('Y-m-d',$time);
$enddate = date('Y-m-d',$time2);

//update the field values and save the updates
$new_user_request -&gt; field_start['und'][0]['value'] = $startdate;
$new_user_request -&gt; field_end['und'][0]['value'] = $enddate;
user_save($new_user_request);


Hopefully this helps someone else too.