Following on from my comment on @RichardAbey-Nesbit's answer, I propose:
least imminent
Allow me to state my reasoning. The obvious concise answer to this question if it referred to a date furthest in the past would be least recent.
We are accustomed to ask "how recent?", and to talk of something being "very recent" (something happening emphatically not long ago), "less recent" (something happening a comparatively longer time ago), and "not recent" (something happening in the distant past, or expressing doubt that it happened at all).
And it is apparent that the future-oriented counterpart word is imminent, and can be used in exactly the same combinations like "very imminent" (about to happen emphatically soon), "less imminent" (something that will happen comparatively further in the future), and "not imminent" (something that will happen in the distant future, or expressing doubt that it will occur at all).
So given a set of dates, the date furthest in the future is the least imminent.
The phrase does not seem idiomatic, so it may give people pause for thought when first encountered, but it does not involve any new coinage, and there is no possible ambiguity in meaning (for example, I can't see any train of thought that would lead a person to think "least imminent" meant the soonest rather than latest date in the future, or that it referred to any point in the past).