The Sword of Aradel — Alexander Key

To save his kingdom, a stable boy steps through a portal to the future

When the duke dies, the evil wizard Albericus presents his heir with the sword of Aradel, a magical blade whose owner can never be bested in combat. Brian, the stable boy, has no interest in politics until the day that he spills a water trough onto the new duke's horse and finds the sword of Aradel pointed at his throat. He picks up his quarterstaff, expecting certain death, but the lowly stable boy defeats the duke with ease. The sword of Aradel is a fake!
 
The true sword has been hidden by spell in a far-off future land. A powerful sorceress knights Brian and sends him forward in time to find the blade that will save the kingdom. With Albericus in pursuit, Brian charges through the portal and emerges in a strange new world known as New York City, where a magic sword waits for a noble knight to wield it once again.