After getting a subscription (as shown in the sample code), the startDate is available with

$response->getSubscription()->getStartDate();

because the value is populated without hours, minutes, or seconds, PHP uses the current hours, minutes, and seconds (see discussion at PHP.net).

Therefore the startDate as a timestamp varies based on when it is retrieved, it would be great if this could be populated with 00:00:00 for hours, minutes, and seconds (or any other constant value) so the startDate would be consistent regardless what time of day it is retrieved.