This is a really simple mod, don't take this as fully working one, au contraire.

This will most surely break things that require day time.

It works by removing one tick of the time x% of the time.
E.g. at 2x day duration, 50% of the time a tick is removed. As a matter of fact the time increase is not precise.

The sky will 100% flicker back and forth.

There's a config to set separately day duration and night duration. By default, Day duration is 2x and Night 1.5x.

Just want to clarify that I'm not going to fix the below bugs, as said, this mod is really simple and I'm not going to mess with MC's day/light cycle.

Known mods / mechanics that break

Minecolonies (reported by sprayartrick)

There are a few oddities, but nothing major.
Player sleeping doesn't advance to the next morning despite it saying "Good morning".
Things still progress as normal but events that are suppose to last a whole day or several days will count out multiple days in a single longerdays cycle. For example, colony deaths result in the colony mourning for their fallen colonist the next day, halting all work. My 2 hour day/night cycle entails about 6 days of vanilla time. Mourning colonists go back to work after about 20 minutes rather than waste the whole day because for them a full day/night cycle as passed.

Optifine and/or Shaders (thanks to misterchester67)

Seems like chunks keep reloading (like when you press F3+A) continuously.