What are peoples feelings on if diameter can be used for an environmental trigger on an item? My first instinct was sure it ends a spells duration so why shouldn't it work?
Then when I thought about it diameter duration is a little different from other celestial durations like sun, moon, and year. A sun duration spell could last hours or only minutes depending on when it's cast (or weeks if cast far enough north at the right time of year). How close you are to sunrise or sun set has no effect on the difficulty and energy you put into the spell. It's something about the sun crossing the horizon that snuffs the spell. Moon and year spells also have the ability to vary significantly in their actual duration. Moon by 27 days year by up to almost three months.
Diameter is different its based on the actual time that passes rather then a specific event, two spells cast 60 seconds apart will end sixty seconds apart. Sure it's the time it takes for the sun to move it's diameter in the sky but that not a specific event the sun is always moving. It is perpetualy just finishing and about to start this duration. The sun The sun doesn't even have to be up to cast spells with this duration. Their is no magic moment when everything changes. Spells with this duration each have their own clock. So is it still enough to hang an environmental trigger on.
What about momentary then could you build a environmental trigger that goes off every round? Could you build a trigger around Ring duration?
Or think about all the nonstandard durations a magus might learn. Like the ones available to Merinita Year+1, Bargain, Fire, or Until. Bargain and Until scare me a little as environmental triggers. What's to stop a Merinita making any event a trigger.
This is why I wouldn't accept Diameter. It's not an automatically noticeable change to the environment.
Could you set up something to do it? Yes. But I would make you do it the hard way. For example, use an Concentration duration InVi effect to check a Diameter duration CrVi effect that has a linked trigger to the InVi effect. Meanwhile place other effects in the device that are also linked to the InVi effect. Is that a pain? Sure. But if you want something automatically firing off like this, then I would expect you to work for it.
As long as we're being pedantic and not picky about stuff, Year can last as little as 6 months and a day or as long as a year. That's almost six months not almost three
As to the main question I don't think that diameter is "a feature of the items environment" so I wouldn't allow it. On the other hand there's no reason that the environmental trigger has to be linked to a spell duration, only "major magical features". I won't say that there isn't a fair amount of ambiguity about what is a major magical feature and what is the sort of thing that needs an intellego spell and a linked trigger, but here are a few of the things that I'd use:
I think that an environmental trigger could be linked to the swing of a pendulum (if the item were on or contained the pendulum).
It could target to being submerged in water, so you could run duration diameter off of something like a water wheel in a stream (when it's not frozen over).
You could make the effect trigger whenever wind blows on the item from the north.
If the item were set within a room I might allow it to trigger whenever the door opened.
How so? I think maine75man was correct. A Year Duration spell lasts the time to the next solstice/equinox PLUS three seasons (one season between the first and the second equinox/solstice after the casting; a second season between the second and the third; and a third season between the third and the fourth). So the variability is a single season, not two.