Stripe API - Next Payment date

$timestamp = "2306060846";
$next_payment_date = date('Y-m-d',$timestamp));

It gives you time in Y-m-d format

You can check the status of the subscription as follows:

customer = Stripe::Customer.retrieve("cus_7G9REJXtaW05QY")
subscription = customer.subscriptions.retrieve("sub_7HFIqkWIDqEhho")

if subscription.status == 'trialing'
  next_payment_date = Time.at(subscription.trial_end).strftime("%B %d, %Y")
end

After the trail ends you can check the current_period_end attribute from the subscription

next_payment_date = Time.at(subscription.current_period_end).strftime("%B %d, %Y")

Moreover, you can use the current_period_end if you have only one month trial. That would work in all the cases.

PS: For the status check, the word is trialing and not trialling, if I am not wrong, there is a spelling mistake by Stripe team. :-)

trial_end gives the next_payment_date in timestamp.

You can transfer it into date format using date function in php.

Update: As of mid 2019, for a subscription not currently in trial, you'll find the Unix timestamp for the next billing period in the Subscription object as current_period_end.

As far as I know, it is not fully possible. trial_end (if set) will be the first payment date. current_period_end will be the next charge attempt, but if there is a decline, the next charges will follow the schedule in the settings (e.g. retry after 1 day, retry after 3 days, cancel). You must track declines and to calculate the next payment date using the rules from the settings.