2054:
In the Justice League Unlimited episode "Epilogue", which is set in 2069 (65 years after previous JLU episode: "Divided We Fall"), Terry McGinnis imagines a conversation with his girlfriend Dana Tan in which she mentions they've been dating for 15 years.
"Sure. Why else keep dating for fifteen years?" - Dana Tan
2069 - 15 = 2054.
They had been dating from at least 2040-2042 so they must have broken up at some point after that and reunited in 2054 for the reference to make sense.