I had the very same issue. I ended up pulling the app events, getting the installation date and billing_on and creating a subscription model by adding 30 days to every period. but be aware that if you give 0 trial days, the billing_on is at install date + 30 days, for no reason apparently.
Long story short, Shopify does not really invest in the partner API and you have to find creative solutions. For example, it would be very cool to have the transaction period start date and end date, like today you have a transactions, which period is that related to? Or the trial days, inside the event.