I recently had at least two workunits that reported very high execution times, but that didn't actually run so long. See this, for example: http://moowrap.net/result.php?resultid=30220457 or http://moowrap.net/result.php?resultid=30226010

In the first case you may see: Run time 12 hours 26 min 3 sec, but the task started at 12:24:59 CEST (10:24:59 UTC) and ended at 12:59:14 CEST (10:59:14 UTC) thus running normally (~30 minutes for the HD6950).

Something should be wrong in date/time calculations, maybe time zones issues.