I made one on Excel
I’ve been counting down by monthly meetings. Two each month, and now it’s extra sweet whenever one is cancelled.
I never needed or had use for a visual reminder myself.
My retirement was based essentially on time served (i.e. a defined benefit pension). But it also was impacted by on-going contract negotiations that lasted almost a year and impacted future non-retroactive retirement medical benefit increases - I needed to retire after a specific date after the contract was signed for them to apply. Meanwhile I had to calculate how much vacation time I’d have on the book and plan when best to burn it. Vacation was more useful taken as actual vacation paid w/ a shift differential and slightly extending my retirement credits than it was paid out as a heavily-taxed lump sum at base rate after I retired. Also had to factor in my steadily ticking sick leave accumulation which could be used as retirement service extension credits if not taken as sick leave. All this plus my retirement date “had” to be on the first of whatever month so I got my first check at the end of that month, otherwise I’d have to wait until the end of the following month to get paid, which would be…annoying. With all of that I was absolutely gnat’s ass about my retirement date. Calculating and re-calculating constantly as contract negotiations dragged on and all of those little factors shifted minutely over time. Sure I needed the best fiscal decision, but fraying workplace sanity was also a factor 
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A visual counter wouldn’t work because the day kept shifting. But more importantly it wouldn’t have mattered anyway because I was so laser-focused on getting the fuck out that the prime candidate dates were seared into my very soul 
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For me, it’s a countdown to my 30th anniversary. Not an as-yet-undetermined last day of work.
My date was 12/31/2019, so there wasn’t much tracking necessary…just the approach of the holidays was the reminder.
Mine was tied to a birthday. In some sense I knew it in kindergarten.
But all the rest of your variables applied. Including ongoing contract negotiations that could materially alter the terms of the rest of my life. If, and only if, a process that cared nothing of me came to a climax before my own magic day.
Worked out ok, not incredible. Like so much else of life. Good enough and no (or not much) whining from here.
My actual first day of retirement was April 1. Before announcing it to my co-workers, the thought crossed my mind that nobody would believe me, given the date.
mmm
I knew my 30 year date for almost 30 years. I started working in February, and at that time, we got a “pension statement” at the beginning of the fiscal year in March. So that had the specific date of when I’d qualify for a 30 year pension, and, more amusingly, what pension I could get at that time if I had to retire due to disability. Since I’d only worked about a month, that was something silly like $200/year 
So that made an impression on me. Of course, with the slightly early retirement I’ve signed up for, I won’t quite get to that date.
But I’ve just started the fourth-to-last week of work! Got an e-mail today from the pay center that they’ve entered more of my data, so everything is still on track.
Hooray!
Mine was kinda big. I downsized a lot of stuff. A LOT of stuff. But if felt good.
Stuff keeps creeping in, though, so I have to eternally vigilent.
The last few years before I retired, I had so much vacation time (and too much work) so I took every other Friday off. It helped a little.