Adventures in Blackmoor is D&D Expert module DA1 (Dave Arneson 1) and is set in Mystara / Arneson's Blackmoor (assigned as Mystara 3,000 years ago) as opposed to Greyhawk's homage location. The mostly indestructible inn contains both a temporal and dimensional gate though the PCs only experience the temporal aspect first hand, dangerous planar critters leak through demonstrating the other. The entire adventure takes place inside different incarnations of the inn -- at least 3 and more likely 6-8. There are 3 distinct versions and one version you can end up visiting in the recent future/past a few times.