2038+ is the bane of my existence

I don't know if this is supposedly already worked out, but I've run into a problem with the future date of 2038.
I'm using php 5.1 with the php4 module installed.

Here is how I came across the issue.
I'm writing a custom module with some cross field validation and ran into this problem comparing two dates.
Here is my code:

// Make sure we're checking real dates here
if(!is_date($node -> field_birthday[0][value]) || !is_date($node -> field_expected_retirement_date[0][value])){
  // If not, we know the dates aren't right, so return an error
  form_set_error('field_expected_retirement_date', t('The year for the expected retirement date should be the birth year + 65'));
}else{// Otherwise, go ahead and check the difference
  $date_diff = date_difference($node -> field_birthday[0][value], $node -> field_expected_retirement_date[0][value], 'years', $type);
  if($date_diff != 65){//check to make sure the date difference is 65
    drupal_set_message($date_diff . " for whatever reason isn't 65", "error");
    //@TODO: fix this so it only highlights the year.
    form_set_error('field_expected_retirement_date', t('The year for the expected retirement date should be the birth year + 65'));
  }
}

Now if I set the birthday to 1975 and the expected retirement date to 2040 (which should be 65) the value returned from the date_difference function is 72.

The date_difference between 1975 and 2038 is a correct 63, but with 2039 is 73 and counts down every year from there.
That means that the date_difference between 1975 and 2047 return 65.

And just some notes about that code in case you were wondering:

$type is declared outside this code where I figure out what format the dates are in.
I had to do this because the validate hook in this custom module is called after the standard validation. If the standard validation passes, the dates are in ISO format. If it doesn't pass, the dates are in array format. Not sure why this is, but I worked around it.

is_date is my own customized version of date_is_valid.