Fans had to put a lot of work into making the older shows internally consistent with the show's universe and timeline. There was a lot of fan-created supplementary material to stitch things together.
One person who created some of these supplementary materials back in the '70s is currently posting on TrekBBS. I refer to
Mark Andrew Golding, who wrote several articles for
Trek (semi-professional fan magazine published in the '70s and later the best articles were anthologized in a series of paperback books called
The Best of Trek).
I read Golding's articles, and the sheer scope of detail in them is amazing - particularly given that he wrote those articles back in the days before we had VCRs, never mind computers, the internet, or being able to borrow or buy episodes at libraries or video stores (modern fans are damned spoiled by all this and seem incapable of understanding that). He would have had to read copious articles, re-watched the episodes many,
many times (not sure if Bjo Trimble's Star Trek Concordance had been published by the time Golding started doing his articles), and taken a mountain of notes.
When I realized who "MAGolding" was over there, I read all of his posts, and was transported back 40 years to when I'd first read his articles. He only had TOS to work with at the time; I've no idea if he's produced similar output regarding the other series.
I've tried to explain all this to the people there who can't stand his extremely detail-oriented posts (I do suspect he might be more than a bit OCD, but that's normal enough for people in the SF community). His posts are long and exacting, and he outlines every step of his reasoning, and the evidence for it.
And the modern generation of fans over there just can't wrap their heads around it. They have someone who is probably 60+, who started writing about this stuff over 40 years ago, and rather than using the opportunity to ask him questions, they just revert to being trolls.