Downtown Calgary is home to a small network of enclosed walkways (a "skywalk") called the +15 System. The +15's are designed to encourage pedestrian traffic during the day, but are closed at night. The city claims that this is for the sake of the security of the buildings connected by the walkways and to prevent transients from living in them. However, the actual reason is much more interesting.
While the core of the system is open from 7 AM until Midnight, The western and eastern edges of the system are locked at 9 PM. If you are able to sneak into the closed systems at midnight, you'll discover new walkways that don't exist during the daylight hours. If you walk down these pathways, you'll eventually emerge in the Minneapolis Skyway.
The Minneapolis Skyway connects every skywalk system in the world. Be warned, however, that if the skywalk you enter from it isn't in a city where the time is between midnight and six AM, you may find that walkways you rely upon no longer exist.
Note from the Anthologist, Dr. Johnson:
Wikipedia has a listing for skyways/skywalks, and a table of a few of the largest skyway networks in the world. I took the time to plot them on a map, making sure that each point is either specifically on part of a network itself or on one of the buildings connected to one. I noticed that a significant portion of the networks connect up to a city library, which may be important.
One thing I've been thinking on recently is what constitutes a "skywalk system." There are plenty of skywalks or skyways that merely connect one building to another - would you be able to use one of these to get to the Minneapolis system? Or must it be one of the ones that has multiple connected walkways? And suggesting that the Minneapolis Skyway "connects" them all together is somewhat ambiguous. I took it to mean that you'd need to stop off in Minneapolis before moving on to your final destination, this being supported by the possibility that you're potentially crossing a significant number of time zones and would need then to wait until it's between the proper hours at your destination to continue forward.