A span of time in the future equal to the current length of a relationship. For a relationship 6 months long, the Date-Time Continuum's event horizon lies 6 months in the future.
Violating the Date-Time Continuum by making plans beyond the event horizon is ill-advised and stands to cause the relationship to explode in a dazzling supernova of animosity or collapse into a blackhole of mutual loathing.
Ted: "Stella and I are going to her sister's wedding in 6 months"
Barney: "Wait, how long have you and Stella been dating?"
Ted: "3 months"
Barney: "Dude, bad idea. You can't make any plans with Stella for anything more than 3 months in the future. Don't risk violating the Date-Time Continuum."