diff --git a/lib/Config/Migrator.php b/lib/Config/Migrator.php
index 747598d74b..206cc3d7c1 100644
--- a/lib/Config/Migrator.php
+++ b/lib/Config/Migrator.php
@@ -60,9 +60,9 @@ class Migrator {
       'name varchar(90) NOT NULL,',
       'type varchar(90) NOT NULL DEFAULT "default",',
       'description varchar(250) NOT NULL DEFAULT "",',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'deleted_at TIMESTAMP NULL DEFAULT NULL,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'deleted_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY name (name)'
     );
@@ -110,11 +110,11 @@ class Migrator {
       'count_processed mediumint(9) NOT NULL DEFAULT 0,',
       'count_to_process mediumint(9) NOT NULL DEFAULT 0,',
       'count_failed mediumint(9) NOT NULL DEFAULT 0,',
-      'scheduled_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'processed_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
-      'deleted_at TIMESTAMP NULL DEFAULT NULL,',
+      'scheduled_at TIMESTAMP NULL,',
+      'processed_at TIMESTAMP NULL,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'deleted_at TIMESTAMP NULL,',
       'PRIMARY KEY  (id)',
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -128,9 +128,9 @@ class Migrator {
       'last_name tinytext NOT NULL DEFAULT "",',
       'email varchar(150) NOT NULL,',
       'status varchar(12) NOT NULL DEFAULT "unconfirmed",',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'deleted_at TIMESTAMP NULL DEFAULT NULL,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'deleted_at TIMESTAMP NULL,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY email (email)'
     );
@@ -143,8 +143,8 @@ class Migrator {
       'subscriber_id mediumint(9) NOT NULL,',
       'segment_id mediumint(9) NOT NULL,',
       'status varchar(12) NOT NULL DEFAULT "subscribed",',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY subscriber_segment (subscriber_id,segment_id)'
     );
@@ -157,8 +157,8 @@ class Migrator {
       'subscriber_id mediumint(9) NOT NULL,',
       'custom_field_id mediumint(9) NOT NULL,',
       'value varchar(255) NOT NULL DEFAULT "",',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY subscriber_id_custom_field_id (subscriber_id,custom_field_id)'
     );
@@ -176,9 +176,9 @@ class Migrator {
       'reply_to_name varchar(150) NOT NULL DEFAULT "",',
       'preheader varchar(250) NOT NULL DEFAULT "",',
       'body longtext,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'deleted_at TIMESTAMP NULL DEFAULT NULL,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'deleted_at TIMESTAMP NULL,',
       'PRIMARY KEY  (id)'
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -192,8 +192,8 @@ class Migrator {
       'body LONGTEXT,',
       'thumbnail LONGTEXT,',
       'readonly TINYINT(1) DEFAULT 0,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id)'
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -204,8 +204,8 @@ class Migrator {
       'id mediumint(9) NOT NULL AUTO_INCREMENT,',
       'name varchar(90) NOT NULL,',
       'newsletter_type varchar(90) NOT NULL,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY name_newsletter_type (newsletter_type,name)'
     );
@@ -218,8 +218,8 @@ class Migrator {
       'newsletter_id mediumint(9) NOT NULL,',
       'option_field_id mediumint(9) NOT NULL,',
       'value varchar(255) NOT NULL DEFAULT "",',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY newsletter_id_option_field_id (newsletter_id,option_field_id)'
     );
@@ -231,8 +231,8 @@ class Migrator {
       'id mediumint(9) NOT NULL AUTO_INCREMENT,',
       'newsletter_id mediumint(9) NOT NULL,',
       'segment_id mediumint(9) NOT NULL,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY newsletter_segment (newsletter_id,segment_id)'
     );
@@ -246,8 +246,8 @@ class Migrator {
       'queue_id mediumint(9) NOT NULL,',
       'url varchar(255) NOT NULL,',
       'hash varchar(20) NOT NULL,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id)',
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -260,9 +260,9 @@ class Migrator {
       'body longtext,',
       'settings longtext,',
       'styles longtext,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'deleted_at TIMESTAMP NULL DEFAULT NULL,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'deleted_at TIMESTAMP NULL,',
       'PRIMARY KEY  (id)'
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -274,7 +274,7 @@ class Migrator {
       'newsletter_id mediumint(9) NOT NULL,',
       'subscriber_id mediumint(9) NOT NULL,',
       'queue_id mediumint(9) NOT NULL,',
-      'sent_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'sent_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id)',
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -288,8 +288,8 @@ class Migrator {
       'queue_id mediumint(9) NOT NULL,',
       'link_id mediumint(9) NOT NULL,',
       'count mediumint(9) NOT NULL,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
-      'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
+      'created_at TIMESTAMP NULL,',
+      'updated_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id)',
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -301,7 +301,7 @@ class Migrator {
       'newsletter_id mediumint(9) NOT NULL,',
       'subscriber_id mediumint(9) NOT NULL,',
       'queue_id mediumint(9) NOT NULL,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
+      'created_at TIMESTAMP NULL,',
       'PRIMARY KEY  (id)',
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -313,7 +313,7 @@ class Migrator {
       'newsletter_id mediumint(9) NOT NULL,',
       'subscriber_id mediumint(9) NOT NULL,',
       'queue_id mediumint(9) NOT NULL,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
+      'created_at TIMESTAMP NULL,',
       'PRIMARY KEY  (id)',
     );
     return $this->sqlify(__FUNCTION__, $attributes);
@@ -324,7 +324,7 @@ class Migrator {
       'id mediumint(9) NOT NULL AUTO_INCREMENT,',
       'form_id mediumint(9) NOT NULL,',
       'subscriber_id mediumint(9) NOT NULL,',
-      'created_at TIMESTAMP NOT NULL DEFAULT 0,',
+      'created_at TIMESTAMP NULL,',
       'PRIMARY KEY  (id),',
       'UNIQUE KEY form_subscriber (form_id,subscriber_id)'
     );
diff --git a/lib/Subscribers/ImportExport/Export/Export.php b/lib/Subscribers/ImportExport/Export/Export.php
index 93e1292e3b..c3d7ebeed4 100644
--- a/lib/Subscribers/ImportExport/Export/Export.php
+++ b/lib/Subscribers/ImportExport/Export/Export.php
@@ -175,6 +175,7 @@ class Export {
   }
 
   function getSubscribers($offset, $limit) {
+    // JOIN subscribers on segment and subscriber_segment tables
     $subscribers = Subscriber::
     left_outer_join(
       SubscriberSegment::$_table,
@@ -192,6 +193,8 @@ class Export {
         ))
       ->filter('filterWithCustomFieldsForExport');
     if($this->subscribers_without_segment !== false) {
+      // if there are subscribers who do not belong to any segment, use
+      // a CASE function to group them under "Not In Segment"
       $subscribers = $subscribers
         ->selectExpr('CASE WHEN ' . Segment::$_table . '.name IS NOT NULL ' .
                      'THEN ' . Segment::$_table . '.name ' .
@@ -204,21 +207,26 @@ class Export {
           $this->segments
         );
     } else {
+      // use an aggregate function to prevent non-deterministic GROUP BY issue
+      // in MySQL 5.7+
       $subscribers = $subscribers
-        ->select(Segment::$_table . '.name', 'segment_name')
+        ->selectExpr('CONCAT('.Segment::$_table . '.name) as segment_name')
         ->whereIn(SubscriberSegment::$_table . '.segment_id', $this->segments);
     }
     if(!$this->group_by_segment_option) {
+      // if grouping by segments, use a GROUP BY clause on subscriber id
       $subscribers =
         $subscribers->groupBy(Subscriber::$_table . '.id');
     }
     if($this->export_confirmed_option) {
+      // select only subscribers with "subscribed" status
       $subscribers =
         $subscribers->where(Subscriber::$_table . '.status', 'subscribed');
     }
     $subscribers = $subscribers
       ->whereNull(Subscriber::$_table . '.deleted_at')
-      ->limit(sprintf('%d, %d', $offset, $limit))
+      ->offset($offset)
+      ->limit($limit)
       ->findArray();
     return $subscribers;
   }
diff --git a/tests/unit/Models/CustomFieldTest.php b/tests/unit/Models/CustomFieldTest.php
index 647fd3cf54..a3e7df2cea 100644
--- a/tests/unit/Models/CustomFieldTest.php
+++ b/tests/unit/Models/CustomFieldTest.php
@@ -66,7 +66,6 @@ class CustomFieldTest extends MailPoetTest {
   function testItHasACreatedAtOnCreation() {
     $custom_field = CustomField::findOne($this->custom_field->id);
     expect($custom_field->created_at)->notNull();
-    expect($custom_field->created_at)->notEquals('0000-00-00 00:00:00');
   }
 
   function testItHasAnUpdatedAtOnCreation() {
diff --git a/tests/unit/Models/FormTest.php b/tests/unit/Models/FormTest.php
index 718326cc56..68c4a947a0 100644
--- a/tests/unit/Models/FormTest.php
+++ b/tests/unit/Models/FormTest.php
@@ -49,7 +49,6 @@ class FormTest extends MailPoetTest {
   function testItHasACreatedAtOnCreation() {
     $form = Form::findOne($this->form->id);
     expect($form->created_at)->notNull();
-    expect($form->created_at)->notEquals('0000-00-00 00:00:00');
   }
 
   function testItHasAnUpdatedAtOnCreation() {
diff --git a/tests/unit/Models/NewsletterOptionFieldTest.php b/tests/unit/Models/NewsletterOptionFieldTest.php
index b57a280387..1e6ef9dd86 100644
--- a/tests/unit/Models/NewsletterOptionFieldTest.php
+++ b/tests/unit/Models/NewsletterOptionFieldTest.php
@@ -59,7 +59,6 @@ class NewsletterOptionFieldTest extends MailPoetTest {
 
   function testItHasACreatedAtOnCreation() {
     expect($this->option_field->created_at)->notNull();
-    expect($this->option_field->created_at)->notEquals('0000-00-00 00:00:00');
   }
 
   function testItHasAnUpdatedAtOnCreation() {
diff --git a/tests/unit/Models/NewsletterTest.php b/tests/unit/Models/NewsletterTest.php
index 4ff9ab77a0..69a544e7cc 100644
--- a/tests/unit/Models/NewsletterTest.php
+++ b/tests/unit/Models/NewsletterTest.php
@@ -62,7 +62,6 @@ class NewsletterTest extends MailPoetTest {
   function testItHasACreatedAtOnCreation() {
     $newsletter = Newsletter::findOne($this->newsletter->id);
     expect($newsletter->created_at)->notNull();
-    expect($newsletter->created_at)->notEquals('0000-00-00 00:00:00');
   }
 
   function testItHasAnUpdatedAtOnCreation() {
diff --git a/tests/unit/Models/SegmentTest.php b/tests/unit/Models/SegmentTest.php
index 0f711abc9f..683aba9bc8 100644
--- a/tests/unit/Models/SegmentTest.php
+++ b/tests/unit/Models/SegmentTest.php
@@ -78,7 +78,6 @@ class SegmentTest extends MailPoetTest {
   function testItHasACreatedAtOnCreation() {
     $segment = Segment::findOne($this->segment->id);
     expect($segment->created_at)->notNull();
-    expect($segment->created_at)->notEquals('0000-00-00 00:00:00');
   }
 
   function testItHasAnUpdatedAtOnCreation() {
diff --git a/tests/unit/Subscribers/ImportExport/Export/ExportTest.php b/tests/unit/Subscribers/ImportExport/Export/ExportTest.php
index f8aa865e67..8b07008d6a 100644
--- a/tests/unit/Subscribers/ImportExport/Export/ExportTest.php
+++ b/tests/unit/Subscribers/ImportExport/Export/ExportTest.php
@@ -64,9 +64,9 @@ class ExportTest extends MailPoetTest {
       $entity->hydrate($subscriber);
       $entity->save();
     }
-    foreach($this->segments_data as $custom_field) {
+    foreach($this->segments_data as $segment) {
       $entity = Segment::create();
-      $entity->hydrate($custom_field);
+      $entity->hydrate($segment);
       $entity->save();
     }
     foreach($this->custom_fields_data as $custom_field) {