Cashier: Add trial end date when creating a subscription

I am playing around with Cashier. A user can sign up without a credit card, for a 30 day free trial. I am manually setting the trial_ends_at date in the db when the user signs up. Later on when they choose a subscription, I am creating it like this:

$user->subscription('monthly')
     ->quantity($qty)
     ->trialFor($trail_ends_at)
     ->create($token);

This works great, except for when the trial date has expired. I get an error:

Stripe_InvalidRequestError in ApiRequestor.php line 147:
Invalid timestamp: must be an integer Unix timestamp in the future.

So my only solution is to manually check if the trial has expired and have 2 different create user subscription functions. Is there any way around this?