Is "Attack on Titan" taking place in the past or in the future? According to the prehistory timeline on Wikia,


  
  1003 B1: As a young girl, Ymir Fritz gains the power of the Titans from an unknown source, becoming the Founding Titan. With her powers, she is able to turn her subjects into Pure Titans.
  
  The Fritz family comes into power as the royal bloodline of Eldia, destined to remain unbroken for the next 2,000 years.
  990 B1: For unknown reasons, Ymir Fritz dies 13 years after inheriting her power. Her "soul" is split between Nine Titans that are inherited by nine Subjects of Ymir, with the Founding Titan belonging to the Fritz family and the eight remaining Titans leading clans subservient to the Founding Titan. Those who inherit the power of the Titans are destined to die after 13 years as Ymir did due to a phenomena called the "Curse of Ymir."
  
  The Nine Titans wage war against the nation of Marley, the dominant nation of the land, taking control of the continent and establishing the empire of Eldia. Eldia conquers countless other civilizations as well using Pure Titans as cheap weapons of mass destruction. This results in massive casualties and entire cultures being wiped out.
  Unknown: Eventually, in-fighting breaks out between the various Eldian families for control over the eight Titans, but the Fritz family, using the power of the Founding Titan, maintains balance.
  
  The Ackerman family, an Eldian bloodline resulted from the Eldian Empire's fiddling with the Subjects of Ymir, becomes the sword and shield of the royal family, remaining close to Eldia's king for generations.
  
  Eldia becomes allies with Hizuru.
  


When the Titans arrived on Earth, what year is it?
Let’s say in my opinion, Attack on Titan takes place currently in the year 2350...

Also, "X" stands for "when Titans arrived":


Prehistory (Beginning of time until Zero Before X)
990 years Before X - the Great Titan War begins
Zero years Before X - the Titans arrived


History (Zero Before X to 835 After X)


743 Years After X (2243) - the Great Titan War ends
835 Years After X (2335) - Eren and Mikasa are born in Wall Rose


The story of Attack on Titan (835 to 854 After X)


845 Years After X (2345) - Wall Maria is invaded by the Titans
850 Years After X (2350) - Attack on Titan takes place
851 Years After X (2351) - Wall Maria is set out for the First Time
854 Years After X (2354) - The Future of Attack on Titan