The default value of timescale in Skyrim SE is 20, which means 1 minute in real life, counts as 20 minutes in game.
With this mod 1 minute in real life, counts as 6 or 10 minutes, so time in-game passes more slowly and the days (and nights) last longer.
Installation: Use NMM or drop either timescale6 or timescale10 in your Data folder
Thanks to Old Nick I've added a line in the mod (version 1.1) so the change in value is constant. No more need to activate it via the console on an existing save. In other words, it works out of the box.