I realize PBR isn't 'ready for prime time' but I was experimenting and thought I would relay something I found. I haven't yet tested with the LL viewer. It seems the images of the sun and moon do not update their position as the region/parcel time advances (but lighting does). Selecting 'Refresh visibility of objects' will cause them to jump into their proper position, but they remain there until refreshing again. I noticed in a self built 1.31.0 version of the viewer with details as follows:
*Update: Testing with Second Life Release 7.0.0.582103 (64bit) in Wine does not exhibit this behavior, I can see the moon image (because it's dimmer) incrementally moving through the sky.