I tend to think the start year would be the invalidation of the Holy Sacrifice.
I agree that such makes sense, at least from our limited perspective. In such a case, we'd be looking at either the NOM dates, possibly 1969, or the days surrounding the vernacular usages in the pre-NOM days, such as October 1967, etc. I admit that 1969 to 2039, for example, would be harder to grasp/accept because then we'd be facing 14 more years of trial. God's ways are not our ways and we don't deserve His assistance whatsoever, but it is also difficult to see how the rapidly-disintegrating world situation could even hold together that long. The final, third part of the Secret of Fatima was supposed to be released no later than 1960, so it is also entirely sensible to look at that period as the beginning of the Great Apostasy, etc.
God willing, we will find out soon enough. May we both accept and even willingly embrace His adorable will, whatever it happens to be, now and always. Godspeed.