so... anyone playing the game in 2019 or 2020, will not be able to see that event?
They still can, but only if they lie about the date when they set the clock with the goal of triggering that particular event. (It's the same way with the Japanese version.)
Could the secret event be re-programmed to be dynamic such as a month or two after the date selected during initialization so that everyone has a chance to experience it and the rom won’t need to be changed after 2018?
No, it's more complicated than that... The "event" takes place at very specific (and very short) spans of times in a few locations. Doing so would not be an accurate recreation of the game, which is the goal for this project.
Most people will not trigger it. Most Japanese people missed it, too. But even if you trigger it, you're not likely to be all that impressed. The significance will go straight over the heads of 99% of the people playing it in English. Don't get your hopes about it. If you miss it, you've essentially missed nothing.