diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index e29082ea42..ec8eb4c4a8 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -270,6 +270,8 @@ phutil_register_library_map(array(
     'DifferentialRevertPlanFieldSpecification' => 'applications/differential/field/specification/DifferentialRevertPlanFieldSpecification.php',
     'DifferentialReviewRequestMail' => 'applications/differential/mail/DifferentialReviewRequestMail.php',
     'DifferentialReviewedByFieldSpecification' => 'applications/differential/field/specification/DifferentialReviewedByFieldSpecification.php',
+    'DifferentialReviewerStats' => 'applications/differential/stats/DifferentialReviewerStats.php',
+    'DifferentialReviewerStatsTestCase' => 'applications/differential/stats/__tests__/DifferentialReviewerStatsTestCase.php',
     'DifferentialReviewersFieldSpecification' => 'applications/differential/field/specification/DifferentialReviewersFieldSpecification.php',
     'DifferentialRevision' => 'applications/differential/storage/DifferentialRevision.php',
     'DifferentialRevisionCommentListView' => 'applications/differential/view/DifferentialRevisionCommentListView.php',
@@ -1340,6 +1342,7 @@ phutil_register_library_map(array(
     'DifferentialRevertPlanFieldSpecification' => 'DifferentialFieldSpecification',
     'DifferentialReviewRequestMail' => 'DifferentialMail',
     'DifferentialReviewedByFieldSpecification' => 'DifferentialFieldSpecification',
+    'DifferentialReviewerStatsTestCase' => 'PhabricatorTestCase',
     'DifferentialReviewersFieldSpecification' => 'DifferentialFieldSpecification',
     'DifferentialRevision' => 'DifferentialDAO',
     'DifferentialRevisionCommentListView' => 'AphrontView',
diff --git a/src/applications/differential/stats/DifferentialReviewerStats.php b/src/applications/differential/stats/DifferentialReviewerStats.php
new file mode 100644
index 0000000000..003def828d
--- /dev/null
+++ b/src/applications/differential/stats/DifferentialReviewerStats.php
@@ -0,0 +1,137 @@
+<?php
+
+/*
+ * Copyright 2012 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+final class DifferentialReviewerStats {
+  private $since = 0;
+  private $now;
+
+  public function setSince($value) {
+    $this->since = $value;
+    return $this;
+  }
+
+  public function setNow($value) {
+    $this->now = $value;
+    return $this;
+  }
+
+  public function computeTimes(
+    DifferentialRevision $revision,
+    array $comments) {
+    assert_instances_of($comments, 'DifferentialComment');
+
+    $add_rev = DifferentialComment::METADATA_ADDED_REVIEWERS;
+    $rem_rev = DifferentialComment::METADATA_REMOVED_REVIEWERS;
+
+    $date = $revision->getDateCreated();
+
+    // Find out original reviewers.
+    $reviewers = array_fill_keys($revision->getReviewers(), $date);
+    foreach (array_reverse($comments) as $comment) {
+      $metadata = $comment->getMetadata();
+      foreach (idx($metadata, $add_rev, array()) as $phid) {
+        unset($reviewers[$phid]);
+      }
+      foreach (idx($metadata, $rem_rev, array()) as $phid) {
+        $reviewers[$phid] = $date;
+      }
+    }
+
+    $reviewed = array();
+    $not_reviewed = array();
+    $status = ArcanistDifferentialRevisionStatus::NEEDS_REVIEW;
+
+    foreach ($comments as $comment) {
+      $date = $comment->getDateCreated();
+      $old_status = $status;
+
+      switch ($comment->getAction()) {
+        case DifferentialAction::ACTION_UPDATE:
+          if ($status != ArcanistDifferentialRevisionStatus::CLOSED &&
+              $status != ArcanistDifferentialRevisionStatus::ACCEPTED) {
+            $status = ArcanistDifferentialRevisionStatus::NEEDS_REVIEW;
+          }
+          break;
+        case DifferentialAction::ACTION_REQUEST:
+        case DifferentialAction::ACTION_RECLAIM:
+          $status = ArcanistDifferentialRevisionStatus::NEEDS_REVIEW;
+          break;
+        case DifferentialAction::ACTION_REJECT:
+        case DifferentialAction::ACTION_RETHINK:
+          $status = ArcanistDifferentialRevisionStatus::NEEDS_REVISION;
+          break;
+        case DifferentialAction::ACTION_ACCEPT:
+          $status = ArcanistDifferentialRevisionStatus::ACCEPTED;
+          break;
+        case DifferentialAction::ACTION_CLOSE:
+          $status = ArcanistDifferentialRevisionStatus::CLOSED;
+          break;
+        case DifferentialAction::ACTION_ABANDON:
+          $status = ArcanistDifferentialRevisionStatus::ABANDONED;
+          break;
+      }
+
+      // Update current reviewers.
+      $metadata = $comment->getMetadata();
+      foreach (idx($metadata, $add_rev, array()) as $phid) {
+        // If someone reviewed a revision without being its reviewer then give
+        // him zero response time.
+        $reviewers[$phid] = $date;
+      }
+      foreach (idx($metadata, $rem_rev, array()) as $phid) {
+        $start = idx($reviewers, $phid);
+        if ($start !== null) {
+          if ($date >= $this->since) {
+            $reviewed[$phid][] = $date - $start;
+          }
+          unset($reviewers[$phid]);
+        }
+      }
+
+      // TODO: Respect workdays.
+
+      if ($old_status != $status) {
+        if ($status == ArcanistDifferentialRevisionStatus::NEEDS_REVIEW) {
+          $reviewers = array_fill_keys(array_keys($reviewers), $date);
+        } else if ($date >= $this->since) {
+          if ($old_status == ArcanistDifferentialRevisionStatus::NEEDS_REVIEW) {
+            foreach ($reviewers as $phid => $start) {
+              if ($phid == $comment->getAuthorPHID()) {
+                $reviewed[$phid][] = $date - $start;
+              } else {
+                $not_reviewed[$phid][] = $date - $start;
+              }
+            }
+          }
+        }
+      }
+    }
+
+    if ($status == ArcanistDifferentialRevisionStatus::NEEDS_REVIEW) {
+      $now = ($this->now !== null ? $this->now : time());
+      if ($now >= $this->since) {
+        foreach ($reviewers as $phid => $start) {
+          $not_reviewed[$phid][] = $now - $start;
+        }
+      }
+    }
+
+    return array($reviewed, $not_reviewed);
+  }
+
+}
diff --git a/src/applications/differential/stats/__tests__/DifferentialReviewerStatsTestCase.php b/src/applications/differential/stats/__tests__/DifferentialReviewerStatsTestCase.php
new file mode 100644
index 0000000000..532eb11dae
--- /dev/null
+++ b/src/applications/differential/stats/__tests__/DifferentialReviewerStatsTestCase.php
@@ -0,0 +1,121 @@
+<?php
+
+/*
+ * Copyright 2012 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+final class DifferentialReviewerStatsTestCase extends PhabricatorTestCase {
+
+  public function testReviewerStats() {
+    $revw = DifferentialRevision::RELATION_REVIEWER;
+
+    $revision = new DifferentialRevision();
+    $revision->setDateCreated(1);
+    $revision->attachRelationships(array(
+      array('relation' => $revw, 'objectPHID' => 'R1'),
+      array('relation' => $revw, 'objectPHID' => 'R3'),
+    ));
+
+    $comments = array(
+      $this->newComment(2, 'A', DifferentialAction::ACTION_COMMENT),
+      $this->newComment(4, 'A', DifferentialAction::ACTION_ADDREVIEWERS,
+        array(DifferentialComment::METADATA_ADDED_REVIEWERS => array('R3'))),
+      $this->newComment(8, 'R1', DifferentialAction::ACTION_REJECT),
+      $this->newComment(16, 'A', DifferentialAction::ACTION_COMMENT),
+      $this->newComment(32, 'A', DifferentialAction::ACTION_UPDATE),
+      $this->newComment(64, 'A', DifferentialAction::ACTION_UPDATE),
+      $this->newComment(128, 'A', DifferentialAction::ACTION_COMMENT),
+      $this->newComment(256, 'R2', DifferentialAction::ACTION_RESIGN,
+        array(DifferentialComment::METADATA_REMOVED_REVIEWERS => array('R2'))),
+      $this->newComment(512, 'R3', DifferentialAction::ACTION_ACCEPT),
+      $this->newComment(1024, 'A', DifferentialAction::ACTION_UPDATE),
+      // TODO: claim, abandon, reclaim
+    );
+
+    $stats = new DifferentialReviewerStats();
+    list($reviewed, $not_reviewed) = $stats->computeTimes($revision, $comments);
+
+    ksort($reviewed);
+    $this->assertEqual(
+      array(
+        'R1' => array(8 - 1),
+        'R2' => array(256 - 32),
+        'R3' => array(512 - 32),
+      ),
+      $reviewed);
+
+    ksort($not_reviewed);
+    $this->assertEqual(
+      array(
+        'R1' => array(512 - 32),
+        'R2' => array(8 - 1),
+        'R3' => array(8 - 4),
+      ),
+      $not_reviewed);
+  }
+
+  public function testReviewerStatsSince() {
+    $revision = new DifferentialRevision();
+    $revision->setDateCreated(1);
+    $revision->attachRelationships(array(
+      array(
+        'relation' => DifferentialRevision::RELATION_REVIEWER,
+        'objectPHID' => 'R',
+      ),
+    ));
+
+    $comments = array(
+      $this->newComment(2, 'R', DifferentialAction::ACTION_REJECT),
+      $this->newComment(4, 'A', DifferentialAction::ACTION_REQUEST),
+      $this->newComment(8, 'R', DifferentialAction::ACTION_ACCEPT),
+    );
+
+    $stats = new DifferentialReviewerStats();
+    $stats->setSince(4);
+    list($reviewed, $not_reviewed) = $stats->computeTimes($revision, $comments);
+
+    $this->assertEqual(array('R' => array(8 - 4)), $reviewed);
+    $this->assertEqual(array(), $not_reviewed);
+  }
+
+  public function testReviewerStatsRequiresReview() {
+    $revision = new DifferentialRevision();
+    $revision->setDateCreated(1);
+    $revision->attachRelationships(array(
+      array(
+        'relation' => DifferentialRevision::RELATION_REVIEWER,
+        'objectPHID' => 'R',
+      ),
+    ));
+
+    $comments = array();
+
+    $stats = new DifferentialReviewerStats();
+    $stats->setNow(2);
+    list($reviewed, $not_reviewed) = $stats->computeTimes($revision, $comments);
+
+    $this->assertEqual(array(), $reviewed);
+    $this->assertEqual(array('R' => array(2 - 1)), $not_reviewed);
+  }
+
+  private function newComment($date, $author, $action, $metadata = array()) {
+    return id(new DifferentialComment())
+      ->setDateCreated($date)
+      ->setAuthorPHID($author)
+      ->setAction($action)
+      ->setMetadata($metadata);
+  }
+
+}