diff --git a/CRM/Tbusainvoicegen/Timebank.php b/CRM/Tbusainvoicegen/Timebank.php
index dda479a..49c0026 100644
--- a/CRM/Tbusainvoicegen/Timebank.php
+++ b/CRM/Tbusainvoicegen/Timebank.php
@@ -2,10 +2,11 @@
 
 class CRM_Tbusainvoicegen_Timebank {
 
-   // Live settings
-  const TBBILLABLEFIELD = 'custom_202';
-  const BILLABLEMEMBERFIELD = 'custom_199';
+  // Live settings
+  const TBBILLABLEFIELD = 'custom_233';
+  const BILLABLEMEMBERFIELD = 'custom_236';
   const BILLINGPERIODFIELD = 'custom_228';
+  const TBCREATED = 'custom_112';
 
   // Local dev settings.
   //  const TBBILLABLEFIELD = 'custom_9';
@@ -15,6 +16,7 @@ class CRM_Tbusainvoicegen_Timebank {
   /**
    * Set the pricing here.
    * Up to the number of members in single quotes means you pay the second number every 6 months.
+   * @var array
    */
   private $priceArray = [
     '34' => 30,
@@ -49,6 +51,12 @@ class CRM_Tbusainvoicegen_Timebank {
    */
   private $price;
 
+  /**
+   * The creation date of the timebank.
+   * @var DateTime
+   */
+  private $creationDate;
+
   /**
    * An array of all the contact IDs for whom a contribution already exists in this billing period.
    * @var array
@@ -73,13 +81,14 @@ class CRM_Tbusainvoicegen_Timebank {
   /**
    * Generate invoices (called from API).
    * @param int $cid
+   * @param str $billingPeriod The billing period (e.g "2019-2" for the second 2019 payment)
    * @return array APIv3 standard response.
    */
   public static function generate($cid = NULL, $billingPeriod = NULL) {
     self::setBillingPeriod($billingPeriod);
     // Get a list of contact IDs for everyone to generate an invoice for.
     $contacts = civicrm_api3('Contact', 'get', [
-      'return' => ["id", self::BILLABLEMEMBERFIELD],
+      'return' => ["id", self::BILLABLEMEMBERFIELD, self::TBCREATED],
       'contact_id' => $cid,
       'contact_type' => 'Organization',
       self::TBBILLABLEFIELD => 1,
@@ -89,6 +98,7 @@ class CRM_Tbusainvoicegen_Timebank {
     foreach ($contacts as $k => $contact) {
       if (!in_array($k, self::$contributionExists)) {
         $tb = new CRM_Tbusainvoicegen_Timebank($k, $contact[self::BILLABLEMEMBERFIELD]);
+        $tb->$dateCreated = new DateTime($contact[self::TBCREATED]);
         $tb->setPrice();
         $tb->createContribution();
       }
@@ -127,13 +137,27 @@ class CRM_Tbusainvoicegen_Timebank {
     $adjustedMemberCount = $this->memberCount - 2;
     foreach ($this->priceArray as $members => $cost) {
       if ($adjustedMemberCount <= $members) {
-        $this->price = $cost;
+        $this->price = $cost * $this->monthsProRated();
         break;
       }
     }
+    // Pro-rate the payment if the timebank is new enough.
     return $this->price;
   }
 
+  /**
+   *  If the first annual anniversary of the TB is before the billing period, then the TB pays for the full billing period (6 months).
+   *  If the first annual anniversary of the TB is during any of the first 5 months of the billing period, then the TB pays a pro-rated fee for the number of full billing period months after the first annual anniversary of the TB (1 to 5 months).
+   *  If the first annual anniversary of the TB is during the 6th month of the current billing OR after the billing period ends, then the TB pays $0 for the billing period.
+   */
+  private function monthsProRated() {
+    // By default, we bill all 6 months in a billing period.
+    $monthsProRated = 6;
+    $beginDate = $this->calculatePeriodBeginDate;
+    $firstAnniversary = $this->$creationDate->modify("+1 year");
+    $interval = $beginDate->diff($firstAnniversary);
+  }
+
   private function createContribution() {
     $dueDate = $this->calculateDueDate();
     civicrm_api3('Contribution', 'create', [
@@ -161,4 +185,20 @@ class CRM_Tbusainvoicegen_Timebank {
     return $dueDate;
   }
 
+  /**
+   * Returns the first day of the billing period as a date.
+   * This is to calculate the "new timebank" pro-rated value.
+   * @return Date
+   */
+  private function calculatePeriodBeginDate() {
+    list($year, $number) = explode('-', self::$billingPeriod);
+    if ($number == 1) {
+      $beginDate = new DateTime($year . '-01-01');
+    }
+    else {
+      $beginDate = new DateTime($year . '-07-01');
+    }
+    return $beginDate;
+  }
+
 }