1000 A.D. to 1010 A.D.
From Chrono Compendium
Keystone T-2
-Lucca proposes that time travel is possible with a Time Egg, which utilizes miniature black holes. The first Chrono Trigger was a Time Egg. -Lucca knows of Kid's heritage, somehow. -Lucca composes a letter to Luccia and leaves it with her. *Lucca, at some points, demonstrates the ability to construct incomplete Time Eggs. *Luccia loses her brother in a freak accident in a Porre lab, prompting her to come to El Nido. Note that this accident could be an explosion set by Norris to destroy Grobyc's level seven tech, the StrongArm. However, this may have taken place in either dimension as well, which would explain how an intact arm resides at Chronopolis.
From: Chronology