Consolidate some datetime code and add unit tests

Summary: See D6115.

Test Plan:
  - Ran unit tests.
  - Viewed reports.
  - Created a countdown.
  - Searched chatlog.
  - Searched pastes by created date.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6116

src/applications/search/engine/PhabricatorApplicationSearchEngine.php

@@ -207,18 +207,7 @@ abstract class PhabricatorApplicationSearchEngine {
       return null;
     }
 
-    $viewer = $this->requireViewer();
-
-    $timezone = new DateTimeZone($viewer->getTimezoneIdentifier());
-
-    try {
-      $date = new DateTime($date_time, $timezone);
-      $timestamp = $date->format('U');
-    } catch (Exception $e) {
-      $timestamp = null;
-    }
-
-    return $timestamp;
+    return PhabricatorTime::parseLocalTime($date_time, $this->requireViewer());
   }