I've recently run into an issue where the Rally Connector for HPQC has begun updating items from HPQC to Rally despite the fact that the HPQC item has not been updated.
Example: Defect A in HPQC was last touched in March (according to the History link under Defect Details or the Modified date from the primary defect view). When running the UPDATE_QUALITY CENTER_TO_RALLY service with no <UpdateSelectors>, this defect is returned as needing update even though the time listed is more recent (RallyEIF::WRK::QCConnection.find_updates - Looking for updates made after QC server time of: '2015-08-30 08:33:52' for example).
In short, for some reason the Rally connector for HPQC is updating old HPQC items for what appears to be no reason. I've noticed that if I turn preview mode on and move the date forward and backward I'll get different results, none of which are correct according to the Modified date listed on the defect. There appears to be another date that is being used, but I don't know how to see it to really troubleshoot the issue further.
The updates from Rally to HPQC appear to work without issue. Any assistance would be greatly appreciated!