db_update with a hook
This wouldn't delete just the field, it would delete the row for a user whose uid equals $account-&gt;uid. The -&gt;expression() and subsequent -&gt;execute() as @kiamlaluno points out, wouldn't work right anyway.

If you want to simply delete the value, you are better off doing something like this:

// $account is defined up ^^ there somewhere.
$account-&gt;url_time = '';
user_save($account);