Describing event with “greatest” date value

Introduction

The OP has described a scenario where there is a multiplicity of separate events on different dates, forming a series, such as multiple episodes of a television series. The series of events may be:

1. wholly in the past, including the distant past and/or the immediate past;
2. straddling the past, present and future, i.e. at least one of the events in the series is in the past, one may be in the present, and at least one is in the future;
3. wholly in the future, including the immediate future and/or the distant future.

The OP wants a word (or phrase) to describe the chronologically last event in the series (which he describes as the event having the "greatest (or highest) date-time value"). The chosen word (or phrase) must be suitable for use in all of the situations described in 1 - 3 above.

Choices

A number of different choices of word or phrase have been offered so far in comments and answers. These are:

latest - last - final - next - most recent - most future - farthest (or furthest) date forward

I propose to address these separately, and look at the relevant dictionary definitions of the main contenders.

most future - farthest (or furthest) date forward

Neither of these is suitable if the series of events is wholly in the past, because otherwise the chronologically last event in the series will not be in the future.

most recent

This is suitable only if the series of events is wholly in the past, because otherwise the chronologically last event in the series will be in the future.

next

This is not suitable if the whole series is in the past, or if more than one event in the series is in the future.

last

Dictionary definitions:

last adjective

from Chambers Dictionary
1. being, coming or occurring at the end of a series or after all others.
2. usually applied to dates, time, etc: most recent; happening immediately before the present (week, month, year, etc).
[other definitions not relevant to this question]

from Oxford Dictionaries Online (ODO)
1. coming after all others in time or order; final: [examples omitted]
2. most recent in time; latest: last year

from Merriam-Webster
1. after all others : at the end
2. most lately
3. in conclusion

from Dictionary.com
11. after all others; latest: He arrived last at the party.
12. on the most recent occasion: When last seen, the suspect was wearing a checked suit.
13. in the end; finally; in conclusion.

Merriam-Webster Learner's and Longman add nothing additional of relevance.

Discussion

This was my initial first choice, and indeed the dictionaries all show it to have a primary meaning of:

being, coming or occurring at the end of a series or after all others

which would may it eminently suitable. But it also has a secondary meaning of:

most recent (usually applied to dates, time, etc)

which makes it ambiguous when the series straddles past, present and future, because it could refer to the last episode of the series (in the future), but it could also mean the "most recent" episode (in the past).

latest

Dictionary definitions:

latest adjective

from Chambers Dictionary
1. most recent • the latest update on the news.
2. (the latest) the most recent news, occurrence, fashion, etc.
at the latest not later than a specified time.

Oxford Dictionaries Online (ODO), Merriam-Webster, Merriam-Webster Learner's, Longman, and Dictionary.com all give most recent as the primary or only definition.

Dictionary.com also gives last, and Merriam-Webster gives last (archaic)

Discussion

All the dictionaries give the only relevant meaning of latest as:

most recent

which is the same as the secondary meaning of last, and implies that it must be referring to a past event, which would make it an unsuitable choice for the required purpose.

No dictionaries give the required meaning of "the end of a series", but two do give the meaning of "last", although one indicates that meaning to be archaic.

Nevertheless, @FumbleFingers has argued that latest can be and is used to refer to the chronologically last date in a series of dates falling wholly or partially in the future. He has cited references referring to the latest expiry date, latest expiration date, and latest maturity date.

Although I accept that those usages are appropriate, valid, and seen in legal documents, I would argue that that usage is different from the requirement here. In those cases, there is not a series of events: there is one event (expiry, maturity, etc.) but a choice of dates on which the single event can occur. The terminology there refers to the chronologically last date from the available choices of date: it does not refer to a last event because there is only one event.

In any case, the primary meaning of latest as "most recent" rules it out as a choice here, because of the resultant ambiguity.

final

Dictionary definitions:

final adjective

from Chambers Dictionary
1. occurring at the end; last in a series, after all the others.
2. completed; finished.

from Oxford Dictionaries Online (ODO)
coming at the end of a series

from Merriam-Webster

1. coming at the end : being the last in a series, process, or progress

from Merriam-Webster Learner's

1. always used before a noun
   a. happening or coming at the end

from Longman

1. [only before noun] last in a series of actions, events, parts of a story etc:
   The final episode will be shown tonight.

from Dictionary.com
1. pertaining to or coming at the end; last in place, order, or time

Discussion

Several dictionaries give the primary (relevant) meaning of final as:

occurring at the end
last in a series of events
after all the others
last in time

This makes it not only a suitable choice, but the only remaining choice without some kind of ambiguity. There could be some minor ambiguity over whether it refers to the final episode of a 'current' series, or to the final ever episode (final episode of the final series), but that seems insignificant in the light of the problems with other choices.

Conclusion

The most suitable choice is final.

OP is simply mistaken in thinking latest must imply the events are all in the past. But he's not alone - I've heard people speak of the highest date in computing contexts, to avoid confusion.

But here are hundreds of thousands of 1000s of instances of latest expiry / expiration date, many of which will involves future dates. That would be my preferred option.

If we suppose OP has a list of date values, I certainly wouldn't advise referring to the last date, since that might simply mean the one that comes last in the list, which might not be the latest.