Quoting from the Java Docs:

Unfortunately, as the Earth rotates the length of the day varies. In
addition, over time the average length of the day is getting longer as
the Earth slows.

I am building a new planet and I was wondering exactly this, should the day lengths be fixed or should they vary as my planet rotates?

I do not care about the developers in my planet, are there any reasons to say Unfortunately before the sentence as the .. rotates the length of the day varies.? (I have not decided the name of my planet yet, hence the ...

Actually, I'd argue the opposite; it would be unfortunate in the long term (in geological scales) for life if the length of a day wasn't lengthening.

The primary reason why the length of the day is increasing is because of Tidal Friction, which is caused by the moon. The moon is also getting gradually further away from the earth, and will one day leave Earth's orbit. That's not anticipated to happen before the earth becomes uninhabitable due to the sun increasing in intensity and size, but if it was, it would be a very bad thing for life on earth.

For one thing, the orbit of the moon generates tides and other changes which (aside from seasonal factors) create diversity in the ecosystem that certain animals take advantage of, or at least work to the cycles of. Also, it's thought that the Earth would wobble more without the moon holding it in place, so to speak. The earth already wobbles a little on its axis, causing the Sahara to transition between food basket and desert every few tens of thousands of years, but without the moon it's thought that the earth could eventually drift into a 3 dimensional rotation, creating all sorts of havoc on the surface to different biomes.

In short, the moon does a lot to stabilise the earth while introducing cycles that occur more frequently than seasons. It does that however at a price; tidal friction.

So, it's arguable that most planets with an environment stable enough to form intelligent life, life that can determine its relative fortune according to the environment, probably have a moon. That moon (in turn) is gently robbing the planet of angular momentum, assuming a large body of water. In such a case, intelligent life everywhere is probably experiencing longer and longer days.

I'd argue the best solution is a change in the Java Docs; Unfortunately should be changed to Fortunately.

It really bugs them

What you've probably misunderstood is what the term "Java developer" actually denotes. Java developers are a small mammal similar to civet cats in Indonesia both in physical characteristics and how they are used by humans. They remove bugs from and consume carefully managed Java beans, passing out the undigested remainder into the soil to become source-trees, which is how their common appellation is derived.

As a species Java developers are creatures of habit that are extremely easily disturbed, highly sensitive to changes in their (runtime) environment and typically exhibit signs of OCD. They are so dependent on routine that merely adding or subtracting milliseconds to daytime(and by logical extension, working hours) is enough to cause a marked decline in morale and actual output. This is why in captivity you'll often see them living in production frameworks where their view is carefully modeled and controlled.

Adding a second every 1.5 years or so puts their feeble circulatory systems and nearly nonexistent tolerance under immense stress and is predicted to cause a disastrous decline in their population. This is why it is quietly labelled as "unfortunate" in reference material written by the people who handle Java developers

It is unfortunate, because we don't get to stare at a giant moon all day

If the planet had a moon, and it had a fixed day length, that would mean that it is already tidally locked to its moon. Our moon is tidally locked to Earth, but the reverse is not true. If Earth were also tidally locked to the moon, then day length would no longer change, and one side of the Earth would always be able to see the moon.

Furthermore, in order for two objects to tidally lock each other before the sun turns into a red giant and expands to consume them, they need to be sort of equal in size. That is why Pluto and Charon are locked to each other, but Earth and the moon are not. So in addition to seeing a moon in the sky all day, you would see a HUGE moon in the sky all day. That would be awesome!

Unfortunately, the other side of the Earth would not see the moon at all. Not everyone gets to be a winner.

The earth change in day's length is so small we don't have to care about it, except in real time network systems. Since 1972 (46 years ago) we had 27 seconds of change. Almost one every two years. Any pre internet society would not notice it. So, if you are not a developer...

To make this a issue you need to accelerate the pacing a lot. Say... 10 seconds a year. In 6 years the day would get a minute long, and 360 years would increase it one hour. This would trigger big climate changes and be a major concern.

But to justify this change you would need some radical cosmic event, like the planet capture a errant comet as a new moon. And that would cause some other concerns, like extreme tides too.