Proper way to get "past x years" from argument / altering a date filter with argument handling code

What is the proper way to alter the value of a filter with argument handling code? I'm trying to use an integer argument to specify the number of years back a view should return, e.g., argument of 5 will make the view display the result for "now -5 years." Here's the filter part of my view export:

$handler->override_option('filters', array(
  'type' => array(
    'operator' => 'in',
    'value' => array(
      'graduate' => 'graduate',
    ),
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'id' => 'type',
    'table' => 'node',
    'field' => 'type',
    'relationship' => 'none',
  ),
  'date_filter' => array(
    'operator' => '>',
    'value' => array(
      'value' => array(
        'date' => '',
      ),
      'min' => array(
        'date' => '',
      ),
      'max' => array(
        'date' => '',
      ),
      'default_date' => 'now -3 years',
      'default_to_date' => '',
    ),
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'date_fields' => array(
      'node_data_field_graduation.field_graduation_value' => 'node_data_field_graduation.field_graduation_value',
    ),
    'date_method' => 'OR',
    'granularity' => 'year',
    'form_type' => 'date_text',
    'default_date' => 'now -3 years',
    'default_to_date' => '',
    'year_range' => '-10:+0',
    'id' => 'date_filter',
    'table' => 'node',
    'field' => 'date_filter',
    'relationship' => 'none',
  ),
));

I tried setting various bits of $view->filter based on the argument, but none of it seemed to have any effect--the WHERE clause for the date filter part of the query was already set, and so my changes were ignored. I was able to get my view working by directly overriding the query object's where clause, with the following argument handling code:

$argument = (int) $argument;
$time = date("Y", strtotime("now -$argument years"));
$clause = "DATE_FORMAT(STR_TO_DATE(node_data_field_graduation.field_graduation_value, '%Y-%m-%%dT%T'), '%Y') > '$time'";
$view->query->where['date']['clauses'][0] = $clause;
return TRUE;

...However, this is obviously all kinds of unholy. Is there a flag I need to set that will make views rewrite the where clause based on my changes to the filter object?

And/or, is there a way to do this simply with a date argument, without altering a filter? I initially tried adding a date argument, but it did not seem to have the sorts of options for "time since" that the filter handler did.

Thanks in advance for any assistance.