How to start a stripe subscription with the first 2 months' prorated initial payment upfront

I'm building a checkout page where users sign up for a $100/month product. However, when users sign up, they must pay for the first two months upfront, which must be prorated over time. After the first 2 months, we will ask them to pay 100 USD per month. For example:

Users who register on January 3, 2024, they will have to pay (3-31)/31) * 100 USD for the remaining 28 days of month 1 and 100 USD for the second month.
Starting March 1, 2024, they will pay $100 at the beginning of each month to use

I've seen about subscription schedules, but I don't know how to configure the phase to meet the above requirements.

I tried with the solution in here. How to implement stripe subscription with three months initial upfront payment But it cannot be calculated based on the first month's time rate.

Solution

Step 0 : Create a Test Clock and/or Customer if necessary

const testClock = await stripe.testHelpers.testClocks.create({
    frozen_time: Math.floor(+new Date() / 1000),
  });

  const customer = await stripe.customers.create({
    description: 'test clock customer',
    email : 'example@example.com',
    test_clock: testClock.id,
    payment_method: 'pm_card_amex_threeDSecureNotSupported',
    invoice_settings : {
      default_payment_method : 'pm_card_amex_threeDSecureNotSupported'
    }
  });

Step 1 : Create a Subscription with a 2-month Price, and billing_cycle_anchor set to March 1, 2024

  const subscription = await stripe.subscriptions.create({
    customer: customer.id,
    items: [
      {price:'price_A'} // USD $200 for 2 months
    ],
    payment_behavior : 'allow_incomplete',
    billing_cycle_anchor : 1709251200 // 1 Mar 2024
  }
  );

  console.log(subscription);

Step 2 : Upon successful Subscription first payment, you would create a SubscriptionSchedule for the Subscription

  const schedule = await stripe.subscriptionSchedules.create({
    from_subscription: subscription.id,
  });
  
console.log(schedule);

Step 3 : Update the SubscriptionSchedule to change to a 1-month Price on March 1, 2024

const subscriptionSchedule = await stripe.subscriptionSchedules.update(
    schedule.id, {
      phases:[
        {
          start_date : schedule.phases[0].start_date,
          end_date : schedule.phases[0].end_date,
          items: [{
            price: 'price_A' // USD $200 for 2 months
          }]
        },
        {
          items: [{
            price: 'price_B' // USD $100 for 1 months
          }]
        }
      ]
    }
    );
    
    console.log(subscriptionSchedule);

Advance the test clock to see how it charges