x1 is futureward/to the futureward side/in the future of x2 in frame of reference/according to observer or vantage point x3.
x1 and x2 probably should always be events ("nu"). This word is close to equivalent to "balvi"; a reasonable proposal would be to combine the two words, in analogy with spacial direction brivla; many, if not all, of the available glosses of one probably are shared with the other; minor connotational differences may exist. It means "more recent/more recent side (of event)" if x1 is in the past. In a conceptual universe in which time is not oriented or in which there are multiple time dimensions, or perhaps if time is behaving exotically (such as with time-travel), x3 can also be a "which faces" sumti slot, as in "zunle".