Don't prorate prepaid licenses for the next cycle

I've been working on importing our existing set of website subscriptions into a Commerce License / License Billing backed framework. I talked this out in #drupal-commerce yesterday:

(9:24:31 AM) Kazanir: hey bojanz, is there any existing path to migrating an existing set of subscriptions into a commerce license + recurring setup
(9:24:57 AM) bojanz: Kazanir: no
(9:24:58 AM) Kazanir: i.e. is there a set of functions somewhere I can use to duplicate the Rules functions that create the recurring order for a new license, or am I basically in the Wild West here
(9:31:44 AM) Kazanir: it looks to me like I should be able to use some of the logic in commerce_license_billing_create_recurring_orders -- generate a commerce license entity from the migrating data, put that as the single element of a licenses array, open a billing cycle to match, and then use commerce_license_billing_generate_recurring_order($order, $billing_cycle, $licenses);
(9:32:04 AM) Kazanir: If I am missing a key step anywhere in there, advice would be extremely welcome :)

I proceeded to do this and it more or less worked. I created the license entity using logic similar to the line_item_presave hook, faked a "previous order" entity object, created a matching billing cycle using the logic from the Rules action, and passed those to:

commerce_license_billing_generate_recurring_order($order, $billing_cycle, $licenses);

The problem I had was that the line item for the recurring order would pro-rate itself because of a mismatch between days in months. Since I have a specified end date for the billing cycle (because we are importing existing data), I set the license start date to be one month prior so that it would "match" the next recurring order. I did this with the reverse of the logic from the normal monthly billing cycle:

<?php
  $billing_cycle->end = $expire_ts; // Known expiration date
  $billing_cycle->start = strtotime("-1 month", $expire_ts);
?>

But I'm still hitting a proration issue, as follows:

1. My migrated license has an expiration date of (for example) April 15th. Setting the start date to -1 month gives us March 15th. That means the intervening time is 31 days.

2. The recurring order created has a line item which also specifies a month differential. This becomes April 15th to May 15th. That's 30 days.

This results in commerce_license_billing_prorate_charges() comparing those two against each other and pro-rating the April-to-May charge down to 30/31 (96.8%) of the correct monthly charge.

So I guess I have two questions:

a. Am I doing something wrong, or not understanding the data structure that I should be creating to migrate my existing subscriptions?

b. If not, is this an issue that is experienced during normal usage of asynch monthly license subscriptions?