It's not clear to me what you are referring to.
A D:Sun effect lasts for... D:Sun. You can't stop it (short of dispelling it etc.). The corebook simply notes that if you want an effect that's constantly on, a possible way to achieve it is D:Sun + 2uses/day + environmental trigger (in fact, it turns out to be, in general, the most efficient way).
Eh? I fail to follow you. But note that nowhere does it say that "one must concentrate to maintain the effect of item through the sunrise/sunset". The books says that the effect starts wearing off at sunrise/sunset, so you need to concentrate briefly on it around that time to perpetuate it. I think this fits nicely with the idea that you have a window of opportunity, from when the sunrise begins to when it ends, to "spend" your brief moment of concentration.
No. As I see it if I finish casting the spell/an item produces the effect during the two minutes of sunrise, it lasts until the end of the two minutes of sunset. I fully acknowledge that this part is not made explicit in 5th edition, but I seem to distinctly recall it was clarified in ... some supplement for 4th? and it makes a lot of things work smoothly.
There we are, with an arcane connection to the Dark Lord himself in our hands, and debating about an effect any Verditius would laugh to. Had anyone checked Tolkien's literature to see if anyone were ever using the ring during sunrise or sunset, anyway? I mean, maybe the One Ring actually flickered.
See subsequent discussion on this forum. You may view it as outside of the canon, but it's from the former line editor, by way of callen. That's how you pay for the constant effect, but once it is on, it's on and never goes off and, in fact, cannot be turned off. It's problematic, but it is what it is.
So, it ends at sunrise, and must be recast again. By definition casting time for devices is a round, same as spell casting. A device doesn't have the intelligence to begin casting a spell and then hold the effect until the sunrise has started. I'm not willing to give that power to devices, though you might. As they say, YSMV. But the core rule book does say what I'm saying, just not together, it's a collection of the rules as laid out in multiple places.
Actually, I think that the current "official" position is that it really does get recast 2/day. The magic item that affected its own constant effect with a Muto Vim effect has been changed in the errata to require two uses/day ... which may have lots of justifications, but the most straightforward is that the "constant" effect is just recast 2/day. Note that even in this case the magic item can't be "turned off" since it ineluctably recasts the effect as soon as the previous instance is about to expire.
I still fail to follow you. The way I see it, on an equinox, is this (written in modern terms for clarity):
T=0, beginning of sunrise: the first sliver of the solar disc appear above the horizon. A D:Sun effect whose casting ends now or in the next few hours will last until the end of the following sunset. Any D:Sun effect that was in force during the night will soon fade, but is still in force. Magical devices with environmental triggers "sense" sunrise and proceed to recast their effects.
T=1 Diameter, end of sunrise: at this instant the solar disc becomes visible in its entirety above the horizon. D:Sun effects from the previous night expire now. Devices with appropriate environmental triggers, however, have already recast their effects, which have been in force for about 1 Diameter - 1 round = 19 rounds so far, thus avoiding any flicker.
T=12h, beginning of sunset: the first sliver of the solar disc dips below the horizon. A D:Sun effect whose casting ends now or in the next few hours will last until the end of the following sunrise. Any D:Sun effect that was in force during the day will soon fade, but is still in force. Magical devices with environmental triggers "sense" sunset and proceed to recast their effects.
T=12h+ Diameter, end of sunset: at this instant the last sliver of the solar disc dips below the horizon. D:Sun effects from the previous night expire now. Devices with appropriate environmental triggers, however, have already recast their effects, which have been in force for about 1 Diameter - 1 round = 19 rounds so far, thus avoiding any flicker.
It appears I was arguing from an incorrect understanding of Constant effect items. I went back and reread some things.
Still, your definitions of sunrise and sunset are needlessly complicated and superfluous.
With regards to flicker, it exists, and it is stated as such by the insert on Constant devices, and the fact that items which maintain concentration need to recast the spell as the sunrises or sets. Your definitions of sunrise and sunset appears to make devices which maintain concentration functionally equivalent to Constant effect items, which isn't something I can support.
I insist. No. The insert (and the entire book) never states flicker exists.
The insert states explicitly that flicker does not exist in a situation where flicker would be a problem.
Again, no. The book never states that items which maintain concentration need to recast the effect. In fact, it states the opposite: that by concentrating on the effect, the wielder can perpetuate it, i.e. maintain the effect (possibly cast weeks or years before) without recasting it.
No, though in this case, I'm probably the one to blame for having been unclear. There are several important differences, the two most important ones being:
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Effects with D:Conc + item maintains concentration can be turned off any time; effects with D:Sun last until the next sunrise/sunset regardless of the wielder's wishes (unless they are dispelled, their target is rendered invalid etc.).
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An item with D:Conc + item maintains concentration will become inert by the next sunrise/sunset in the absence active intervention by the wielder; an item with D:Sun + an environmental trigger can be left to itself, and be still operational centuries after it was last wielded.
I had to express them with needless verbosity because I felt I was not sufficiently clear. But they are very simple. Sunrise and sunset are two brief periods of overlap between day and night; anything that lasts "until the next sunrise/sunset" lasts until the end of that brief period.