A friend and I were putting together a shared server, and since we're both Tolkien nerds, we decided we wanted to play as a pair of Unbegotten Elves freshly wandered away from the shores of Cuiviénen. Homo spaiens mode and a variety of extant mods got us most of the way to what we considered an acceptable result, except..... how could we live in an pre-First Age Arda with a bright golden sun or illusive silver moon in the sky?!?!

After far too much meddling with things I did not understand, I finally came upon a very elegant solution: replace the sun and moon textures with transparent pngs, and swap out the sky and sunlight textures with plain black. This removes any visual indication of time passing other than the rotation of the stars!

This, however, will leave you with an exceedingly dark world, or at least visually dark - crops will continue to grow and mobs (hopefully) won't spawn during the "day" (though that is not well tested since I prefer to play without them). I highly suggest you use some other mod like Night Light to raise light levels to compensate so you can, you know, actually see.

(Note that the provided screenshot is using the above Night Light mod at defaul settings - installing just this (and probably your ingame gamma; I don't recall if I've adjusted mine higher than default) will leave you with a much darker world by default.