2032:
How do you know this event takes place in 2032?
Because in the second year of Terry McGinnis's reign as Batman he meets the infiltration unit Zeta:
Batman Beyond episode "Zeta": "Zeta was on a long term assignment, infiltrating a terrorist group; six days ago it went renegade." - Agent Bennet to Police Commissioner Barbara Gordon.
Zeta later had his own series:
The Zeta Project episode "Quality Time": "NSA File Review. Subject: Renegade Infiltration Unit Zeta. Unit Zeta went renegade August 5th, 2041, during the infiltration of suspected terrorist group Brother's Day."
Six Days after August 5, 2041 is August 11, 2041. This information is what grounded Batman Beyond in the 2040's.
2041 (Year 2 of Batman Beyond) minus 1 = 2040 (Year 1 of Batman Beyond).
JLU episode "Epilogue": "I was sixteen years old when you started training me Bruce." - Terry McGinnis in an imagined conversation with Bruce Wayne from "Epilogue".
2040 minus 16 = 2024.
JLU episode "Epilogue": "But when you're making a Batman, genetics is only part of the story; the rest is tragedy. Stop me if you've heard it before. You're 8 years old. Your parents have just taken you to a rousing adventure film; a grand time is had by all. But unknown to you a mysterious figure hides in the shadows. My plan was simple: the killer would leap out at you and kill your family. The trauma would put you on the path to becoming Batman." - Amanda Waller speaking to Terry McGinnis
2024 plus 8 = 2032.