In certain times of the year I can travel 21 hours in time by crossing this 4 km bridge. Where am I?
Another possible answer but didn't found bridge between two place
The islands of Big Diomede and Little Diomede
This is because
Big Diomede is in the easternmost point of Russia and Little Diomede is within the United States' border. They are separated by just two miles (4.8 kilometers) but there is a 20-hour time difference between them.
and, if you cross the
Øresundsbron, a.k.a. Øresund Bridge,
you'll arrive at
the greater Copenhagen area in Denmark.
Because of an outdated historical law, Denmark formally observes mean solar noon time at 15°E longitude, happening to cross the Danish Bornholm island, which is actually further to the East than even Malmö (≈12°E), where you've started your Westward journey! The exact number had been chosen because 15° corresponds to 360°/24, or one hour. This time is close to CEST, UTC+1, but may be offset from it by as much as a second. Beyond that, the UTC is corrected by adding or removing a leap second.
Nobody in fact observes this law, and everyone in Denmark uses CEST precisely. It's interesting, however, that Denmark does not have its own atomic clock in sync with the rest of the world…
Whether it's related to the outdated 19c. law, I don't know.
Time Zone Bridge over Salmon River?
This is because
The Pacific and Mountain time zones are divided along the Salmon River. Thus, there's actually a place where US 95 crosses the river called Time Zone Bridge. Crossing from one side sends you over into the Pacific Time Zone. Crossing from the other side, you enter Mountain Time..