Trying to add the UID or timestamp to GUID doesnt work

I'm trying to ensure a unique GUID is used by various users of feeds. To do this I am attempting to add either a timestamp or their UID to the GUID that they provide, in case two people choose to use the same GUID.

I've tried the following approaches but neither have yielded any results. Field that I am trying to tamper remains unchanged. I'm sure there is something straightforward I am probably missing. Any assistance would be much appreciated. Code snippets below

This is the code I am using to concatenate the current field value with the user uid:

global $user;

  $uid= $user->uid;

  return  $field.$uid;

Alternatively, this is the code I am using to attempt to add a timestamp to the current field value:

$now = time();
return $field.$now;