cast timestamps older than unix epoch to 0

Signed-off-by: Robin Appelman <robin@icewind.nl>

apps/dav/lib/Files/FileSearchBackend.php

@@ -273,7 +273,7 @@ class FileSearchBackend implements ISearchBackend {
 							return max(0, 0 + $value);
 						}
 						$date = \DateTime::createFromFormat(\DateTime::ATOM, $value);
-						return ($date instanceof \DateTime) ? $date->getTimestamp() : 0;
+						return ($date instanceof \DateTime && $date->getTimestamp() !== false) ? $date->getTimestamp() : 0;
 					default:
 						return $value;
 				}