gistfile1.txt

proration date provided = 1488568488 
proration date is just time.now

Returned invoice object:
{
  "object": "invoice",
  "amount_due": 9636,
  "application_fee": null,
  "attempt_count": 0,
  "attempted": false,
  "charge": null,
  "closed": false,
  "currency": "usd",
  "customer": "cus_A6jH2UVlVgF8VO",
  "date": 1488568489,
  "description": null,
  "discount": null,
  "ending_balance": null,
  "forgiven": false,
  "lines": {
    "object": "list",
    "data": [
      {
        "id": "ii_19tLrZGR6JAGMQRJCJ43kVtv",
        "object": "line_item",
        "amount": -364,
        "currency": "usd",
        "description": "Unused time on monthly widgets after 03 Mar 2017",
        "discountable": false,
        "livemode": false,
        "metadata": {
        },
        "period": {
          "start": 1488568488,
          "end": 1489449600
        },
        "plan": {
          "id": "Commence-monthly-widgets-lpZb",
          "object": "plan",
          "amount": 1000,
          "created": 1472840401,
          "currency": "usd",
          "interval": "month",
          "interval_count": 1,
          "livemode": false,
          "metadata": {
          },
          "name": "monthly widgets",
          "statement_descriptor": null,
          "trial_period_days": null
        },
        "proration": true,
        "quantity": 1,
        "subscription": "sub_A6jHj3yBcHY8VR",
        "subscription_item": "si_19mVp0GR6JAGMQRJUnchvbun",
        "type": "invoiceitem"
      },
      {
        "id": "sub_A6jHj3yBcHY8VR",
        "object": "line_item",
        "amount": 10000,
        "currency": "usd",
        "description": null,
        "discountable": true,
        "livemode": false,
        "metadata": {
        },
        "period": {
          "start": 1488568489,
          "end": 1520104489
        },
        "plan": {
          "id": "testyear",
          "object": "plan",
          "amount": 10000,
          "created": 1427340983,
          "currency": "usd",
          "interval": "year",
          "interval_count": 1,
          "livemode": false,
          "metadata": {
          },
          "name": "yearly",
          "statement_descriptor": null,
          "trial_period_days": null
        },
        "proration": false,
        "quantity": 1,
        "subscription": null,
        "subscription_item": "si_19mVp0GR6JAGMQRJUnchvbun",
        "type": "subscription"
      }
    ],
    "has_more": false,
    "total_count": 2,
    "url": "/v1/invoices/upcoming/lines?customer=cus_A6jH2UVlVgF8VO&subscription=sub_A6jHj3yBcHY8VR&subscription_plan=testyear&subscription_proration_date=1488568488"
  },
  "livemode": false,
  "metadata": {
  },
  "next_payment_attempt": 1488572089,
  "paid": false,
  "period_end": 1488568489,
  "period_start": 1488568489,
  "receipt_number": null,
  "starting_balance": 0,
  "statement_descriptor": null,
  "subscription": "sub_A6jHj3yBcHY8VR",
  "subscription_proration_date": 1488568488,
  "subtotal": 9636,
  "tax": null,
  "tax_percent": null,
  "total": 9636,
  "webhooks_delivered_at": null
}

test.rb

require 'stripe'

Stripe.api_key = "test key"
(1..100).each do |i| 
  puts i
proration_date = Time.now.to_i

  # See what the next invoice would look like with a plan switch
  # and proration set:
  invoice = Stripe::Invoice.upcoming(
    :customer => "cus_A6jH2UVlVgF8VO",
    :subscription => "sub_A6jHj3yBcHY8VR",
    :subscription_plan => "testyear", # Switch to new plan
    :subscription_proration_date => proration_date)
  # Calculate the proration cost:
lines = invoice.lines.data[0..-1]#invoice.lines.data.size-2]
  current_prorations = lines.select do |ii|
    if ii.period.start != proration_date
      puts "ERROR"
      puts "=========================="
      puts proration_date
      puts invoice
      puts "=========================="
    end 
    ii.period.start == proration_date
  end 
  cost = 0 
  current_prorations.each do |p| 
    cost += p.amount
  end 
end