UFO Sighting in Denver, Colorado on April 17th 2007 – White crescent shaped object darting around very, very high in the sky

I was sitting in the car with my girlfriend (had three UFO sightings over seven years with this same person). The sun roof was open and we were looking at the sky. I noticed a crescent shaped object darting at unbelievable speeds and constantly changing directions at high speeds. It was far above the clouds, and an airplane flew beneath its flight path, so I could tell that it was very high up, and very large. We both watched the object dart around the sky for about an hour, maybe less, then we drove on.