Is it possible to make one of these for plant growth?? My servers plants grow at twice the speed of vanilla. But the time is incorrect. If it took an hour to grow completely, it'll say it took 2 hours

Maybe possible. Glancing at the Rust code, it looks like there are several networked properties which could be tweaked, such as Age.

How are you influencing the growth speed? Is that a plugin?

The reason Upkeep Display Fix exists as a standalone plugin is that it addresses a shortcoming in Rust which they haven't prioritized fixing. If plant growth speed can only be altered by a plugin, then it would make the most sense for that plugin to alter the display as well (i.e., that plugin should fix the problem it creates).