Correct an issue with epoch timestamps in Conduit

Summary:
Fixes T11375. Some validation code was mishandling raw epoch timestamps.

For numeric values larger than 29999999 (e.g., 2999-12-25, christmas 2999), assume the value is a timestamp.

Test Plan: Used `maniphest.search` to query for `modifiedStart`, got a better result set and saw the `dateModified` constraint in the query.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11375

Differential Revision: https://secure.phabricator.com/D16326

src/applications/search/field/PhabricatorSearchDateField.php

@@ -35,6 +35,14 @@ final class PhabricatorSearchDateField
       return null;
     }
 
+    // If this appears to be an epoch timestamp, just return it unmodified.
+    // This assumes values like "2016" or "20160101" are "Ymd".
+    if (is_int($value) || ctype_digit($value)) {
+      if ((int)$value > 30000000) {
+        return (int)$value;
+      }
+    }
+
     return PhabricatorTime::parseLocalTime($value, $this->getViewer());
   }