Corwin, as Carl Corey, (and later Floramel) spent hundreds of years on a shadow near-indistinguishable from this one: http://en.wikipedia.org/wiki/Earth
Corwin left shadow earth around 1970 AD, as that place reckons time.
Corwin hid the jewel of judgement on Shadow Earth, which further increased its realness.
Time travels roughly 2.5 times faster in Shadow Earth than in Amber, on average. (8 hours in Amber is 20 on Shadow Earth.)
(Since roughly 100 years have passed since Brand shot out Corwin's tire, 250 years have passed on Shadow Earth. This makes the present year ~2220.)