diff --git a/CRM/Tbusainvoicegen/Timebank.php b/CRM/Tbusainvoicegen/Timebank.php
index 08cd2e7..b0dded9 100644
--- a/CRM/Tbusainvoicegen/Timebank.php
+++ b/CRM/Tbusainvoicegen/Timebank.php
@@ -6,9 +6,11 @@ class CRM_Tbusainvoicegen_Timebank {
    * Live settings
    * const TBBILLABLEFIELD = 'custom_202';
    * const BILLABLEMEMBERFIELD = 'custom_199';
+   * const BILLINGPERIODFIELD = 'custom_228';
    */
   const TBBILLABLEFIELD = 'custom_9';
   const BILLABLEMEMBERFIELD = 'custom_10';
+  const BILLINGPERIODFIELD = 'custom_11';
 
   /**
    * Set the pricing here.
@@ -84,9 +86,12 @@ class CRM_Tbusainvoicegen_Timebank {
       'options' => ['limit' => 0],
     ])['values'];
     self::setContributionExists();
-    foreach ($result as $k => $contact) {
-      $tb = new CRM_Tbusainvoicegen_Timebank($k, $contact[self::BILLABLEMEMBERFIELD]);
-      $price = $tb->setPrice();
+    foreach ($contacts as $k => $contact) {
+      if (!in_array($k, self::$contributionExists)) {
+        $tb = new CRM_Tbusainvoicegen_Timebank($k, $contact[self::BILLABLEMEMBERFIELD]);
+        $tb->setPrice();
+        $tb->createContribution();
+      }
     }
   }
 
@@ -103,8 +108,16 @@ class CRM_Tbusainvoicegen_Timebank {
    * Generate an array of contacts who already have a contribution created in this billing period.
    */
   public static function setContributionExists() {
-    // API call to contribution.get to find all records with this billing period
-    // Return an array of contact IDs.
+    $existing = [];
+    $result = civicrm_api3('Contribution', 'get', [
+      self::BILLINGPERIODFIELD => self::$billingPeriod,
+    ]);
+    if ($result['count']) {
+      foreach ($result['values'] as $contrib) {
+        $existing[$contrib['contact_id']] = $contrib['contact_id'];
+      }
+    }
+    self::$contributionExists = $existing;
   }
 
   private function setPrice() {
@@ -121,4 +134,31 @@ class CRM_Tbusainvoicegen_Timebank {
     return $this->price;
   }
 
+  private function createContribution() {
+    $dueDate = $this->calculateDueDate();
+    civicrm_api3('Contribution', 'create', [
+      'financial_type_id' => 'CW License Fee',
+      'date_received' => $dueDate,
+      'total_amount' => $this->price,
+      'contact_id' => $this->cid,
+      'contribution_status_id' => 'Pending',
+      self::BILLINGPERIODFIELD => self::$billingPeriod,
+    ]);
+  }
+
+  /**
+   * Returns the due date as a string.
+   * @return string
+   */
+  private function calculateDueDate() {
+    list($year, $number) = explode('-', self::$billingPeriod);
+    if ($number == 1) {
+      $dueDate = $year . '-03-31';
+    }
+    else {
+      $dueDate = $year . '-09-30';
+    }
+    return $dueDate;
+  }
+
 }
diff --git a/api/v3/Invoicegen/Generate.php b/api/v3/Invoicegen/Generate.php
index b6763ca..33550c8 100644
--- a/api/v3/Invoicegen/Generate.php
+++ b/api/v3/Invoicegen/Generate.php
@@ -11,7 +11,7 @@ use CRM_Tbusainvoicegen_ExtensionUtil as E;
  */
 function _civicrm_api3_invoicegen_Generate_spec(&$spec) {
   $spec['contact_id']['api.required'] = 0;
-  $spec['billing_period']['api.required'] = 0;
+  $spec['billing_period']['api.required'] = 1;
 }
 
 /**