Its supposed to convert timestamps to EXIF format (as that's what the other metadata code expects). Currently its not. Seems to be an issue with failing on the timezone specifier. Possibly it previously worked, but changes to MW's wfTimestamp code caused it to no longer work. Or maybe its always been broken.
Anyhow, minor issue, but should be addressed. Possibly should be addressed by making wfTimestamp handle timezones.