Cashier, adding a column that must be updated on subscription create or resume

I want to provide my users with the ability to lock in their rate for X number of years. However, this is not possible with Stripe API or Cashier. That is why I will have to do it on my application end. To do this I will need a subscribed_at column. I know I can get it straight from Stripe customer's subscription (field current_period_start) but it takes too much time to retrieve it and I will need this functionality quite a lot. So, in my subscriptions table, where I have all of Cashier's needed fields (stripe_id, stripe_active, ..., subscription_ends_at) I want to add this subscribed_at column. Now, I want to automatically update this value anytime a subscription is create, resume, or cancel. On cancel I need to set the value to NULL.

I don't want (and it's not a good practice) to modify Cashier's code. (In StripeGateway.php there is a method updateLocalStripeData() which fires exactly for these three options but as I said I don't want to modify it).

I cannot override the StripeGateway.php method updateLocalStripeData() because my Subscription Model does not extend it.

So far the only solution that worked but which I don't really like is to use Model events saving and there retrieve the subscription's current_period_start field and insert it into my Subscription Model's subscribed_at field.

What other ways to do this?

Update: here is how my plans would look like:

• 2015plan1 $10
• 2015plan2 $20
• 2016plan1 $15
• 2016plan2 $22
• 2017plan1 $20
• 2017plan2 $30

If my user registers in 2015 for plan1 and chooses to lock in their rate for 2 years I only want to change their plan in 2017 for 2017plan1.