After thorough testing of the other mods that promised to change the day/night cycle duration I was severely disappointed at seeing three recurrent issues.

Issues when trying to sleep.
Incompatibility with other mods because how they are coded.
Jittery visuals for the Sun, Moon and Stars.

So is this disappointment that has led me to develop my own day length changer mod. I specifically coded this mod to avoid the aforementioned issues, this below is how I have done it.

The mod has its own internal logic to allow the vanilla sleeping system and in some cases other modded systems (Steve's Realistic Sleep for example) to work as they should.
This mod shouldn't be incompatible with most other mods because it doesn't alter the tick rate or the internal system minecraft uses to keep track of time, exceptuating mods that alter the count of players neccesary for sleeping, as I can´t think of away to make it work without scrapping the already working system. Also there is full compatibility for mods that add sleeping during the day, like Comforts.
Instead of constantly turning on and off the doDaylightCycle gamerule each few ticks, this mod only does it at 12 checkpints along the whole cycle for a duration taken from a calculation based on the total day cycle duration configured. By doing this all jittery visuals are avoided, keepeng stops seamless and transitions fluid.

Also the mod is ressistant to the "/time set" command, as it recalibrates itself every time it is used. If somehow the cycle still breaks, by changing the cycle length for example (I honestly haven't found any other way to wreck it), both at sunrise and sunset the cycle is recalibrated and fixes itself.

Configuration is done through a single gamerule: dayLength

Write: /gamerule dayLength (Time in ticks)

The default vanilla daylight cycle length is 24000 ticks (20 min), so a 48000 ticks (40 min) cycle length is twice as long as a vanilla cycle. In case day length is set to a number equal or lower than 24000, the day will have the normal vanilla day length.

WARNING: If you are using shaders that sync the clouds to the daylight cycle they will also be stopped at the checkpoints, to avoid this find an option that desyncs them. With Complementary Shaders for example:

Install Euphoria Patches > Settings > Configure Euphoria > World > World Time Animation: True

WARNING: If you have tried another day-elongating mods before mine, they might have left your world's ticking system damaged upon uninstall. Sadly I haven't fount any fix to it other than creating a fresh world. In case you uninstall my mod, the only necessary step to take in case of the day-night cycle being frozen is setting the gamerule doDaylightCycle to True, that should permanently fix it to run the vanilla way normally.

Feel free to use this mod in any modpack you want and comment any issues you find, but I won't probably be checking too often, so please be patient.

Kronomancer: Longer Days