Polyurethane clear coat is thickening to a jelly in the can!

I'm assuming oil-based polyurethane. If it's actually water-based, most of this won't apply.

What happened

Since your can of poly was 2/3 filled with air, the remaining finish has partially dried, or cured, in the can due to the presence of a sufficient amount of oxygen, changing its consistency.

What to do this time

You could thin out the partially dry poly with mineral spirits to get it back to its original consistency, but based on my own attempt at doing so, I don't recommend it. Unlike a water-based solution, oil-based polyurethane undergoes a chemical reaction as it dries which is not completely undone by thinning. It would make it possible to apply, but it won't be as good as new. When I tried this, it was a night and day difference in my sprayer between the re-thinned poly and the new can that I opened to complete my job. Your best bet is to buy a new can to finish your cabinets with.

What to do next time

As I said before, the poly reacted to the presence of oxygen in the can. There are a few ways to prevent this from happening. One is to move the leftover poly into a smaller container so that there isn't room in the container for a large amount of oxygen. Mason jars would work well, or anything else with an air tight lid.

Another option is to modify the can it comes in by squeezing it with a clamp.

You can also modify the leftover poly in order to fill up the empty space in the can by adding mineral spirits before sealing the lid. This is a very situational solution, however, as it will change the application performance of the poly when you open it back up. Unless you want to turn it into wipe-on poly or spray poly, you can only add a small amount.

The last option is to remove the oxygen from the can. Since most people don't have a vacuum chamber to store wood finish in, you can displace the oxygen with another gas. I've heard of an aerosol can of inert, heavy gas sold specially for this purpose, though it may be possible to accomplish the same thing with more easily obtained CO2, nitrogen, or hydrogen. You simply spray the gas into the can then immediately seal the lid. The air in the can then has very little oxygen to react with.

I went away for a month and today I opened the can on polyurethane that is about 1/3 full and it appears to be turning to a 'jelly' like consistency. Is this normal

It's not what normally happens with oil-based varnishes once they've been opened but it is one of the things that can occur. It's much more common for varnishes and oil-based paints to skin over in the tin, and underneath the set skin you'll usually find some (sometimes quite a lot) of still-liquid product¹.

Oil-based varnishes and paints 'dry' by two means, the first being evaporation of the solvent (the drying phase) and the second by reacting with oxygen in the air (the curing phase). What has happened here is that the varnish has begun to cure from exposure to oxygen, either because there was enough air within the tin above the varnish or because the seal formed by the lid wasn't perfect so a very slow trickle of air could make its way into the tin, something that occurs fairly frequently.

Can you use it?
Possibly.

It is best to discard the current tin and buy fresh, but depending on how far the gelling has proceeded you may be able to use what remains once or twice more.

If the gel is quite firm it's not really usable any longer, you just won't be able to apply it.

But if the gel is still fairly soft, easily broken up and crushed with light finger pressure, and somewhat wet, the varnish is still just about usable. It can be either spread as-is (you need to work quickly because it will start to tack up much more quickly than previously) or you can attempt to break it down and dilute it somewhat with more mineral spirits/white spirit to get it back to somewhat like its original consistency, but some straining is very likely to be needed (see footnote 1 below on straining).

Avoiding this in future
Obviously the container must have a tight-fitting lid², that's the first priority. So the rim of the tin and the sides of the lid must be kept clean and when the lid is replaced it has to be pressed fully home, if necessary by putting a scrap of wood on top and pressing down with both arms or tapping with a mallet or hammer.

Even if the container has an airtight seal any air trapped inside contains oxygen which will start the curing process, the easiest ways to minimise this are to decant into a smaller container or add something to the original container to raise the liquid level. Glass marbles have long been recommended for this purpose, before that clean pebbles.

Rather than reduce the amount of air that gets trapped inside the container another approach is to replace it with a non-reactive or inert gas. People who have access to compressed nitrogen or CO₂ through their work can use those, the rest of us must use a product such as the aptly named Bloxygen.

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¹ Any remaining liquid varnish is still perfectly usable, but care much be exercised to avoid lumps or specs of set varnish from getting onto the work. The best way to do this is to pierce or peel off the skin, then pour the remaining varnish into a new container through a fine sieve or mesh. Old pantyhose/tights or stocking material make a great sieve for this purpose.

² Each time a lid is prised off there's a chance that the metal can be distorted, which can prevent a tight seal from being possible when it's put back on. So as much as possible try to spread the distortion by using a wider implement (an old spoon can work well) and working around the rim, especially if you find the lid is stuck. Unfortunately it isn't always possible to prevent a lid from distorting badly, some are just so thin you will eventually damage them getting them off. When this happens it's vital if you want to save the rest of the varnish or paint that you decant to a fresh container with a tight-fitting lid or cap. Old, clean, food jars can work well for this.