I agree: whenever you move from day to night you experience sunset, and whenever you move from night to day you experience sunrise.
Thats the point, you DONT experience sunrise/sunset.
You move instantly from day to night. Thats not sunset.
I agree with Direwolf75, sunset/sunrise is the (apparent; although actual I suppose in Mythic Europe) movement of the sun down (or up) over the horizon. When you instantly teleport from day to night you go to a place where sunset has happened more recently than sunrinse --- but you (and the spell) were not there at the precise time of interest.
I disagree!
When you've moved from day to night, the sun has set.
It's really sunset, Occasus, not sunsetting, Occidens.
It's the effect, not the process, despite the fact that we tend to fondly remember the slow sinking of the sun below the horizon.
I think we are hitting a linguistic barrier here.
I also think that my interpretation makes dealing with the issue in game easier 
But hey, if you like it better your way, who am I to gainsay you.
My interpretations.
Which ever comes first, which ever direction you travel. Hermetic magic is tethered to astrological movements. Extreme movement of the spell target would disrupt the spell.
Hermetic magic is tethered to astrological movements. No movement, no tether. Unless the spells get tied to the movements of the point where you entered the regio.
Relative to what? From the individual's perspective, time always moves at the same rate. If the day is shorter than normal, then durations are as well. However if time moves faster. There is no change at all, unless the person leaves midday. Just have to be consistent.
Was the consept of different time in different parts of the world really accepted in medival times?
In any case, the spells fail because these are low phases of magical energy - thus, you would be affected by local time. (unless there were some special rules in the regio you are in...)
Well, almost certainly not by the "common man". But the Greeks certainly did know that the Earth was a sphere, and that the sun at the same "time" would appear in different positions in the sky. In fact, they used the difference in height above the horizon in different positions of the Earth to compute its diameter (with remarkable accuracy).
When we had the Rego discussion, wasn't there something about needing a clear path for Rego to follow? And wouldn't that mean that during that instantaneous travel your path would cross the sunset boundary?
Moreover, there could be some effect that affects this when entering a regio. For instance, when entering a perma-Sun regio, the Sun could move from its mundane position to the regio's, causing an instant sunrise. If you use this interpretation, you'd have to spend the night in the regio to keep your Sun duration spell active.
I just saw a story seed here: a permanent sunrise/sunset area, where Sun duration effects (including parma) only last for a few moments. Would work well to make magi used to be invulnerable to small creatures nervous. You do not even need powerful antagonists to be dangerous for them in that case. A penetration 10 spell to make a magus act like a hen would work great in this context. Or a cock, and have him yelling to the rising sun constantly 
I must put that one somewhere.....
Xavi
All magi are vulnerable for 4 minutes out of every day It would be interesting to see how magi would react to the sun setting every hour... in some weird regio
Some people might houserule that way, but there's nothing (IIRC) that states this is so in RAW. According to RAW you can use ReCo to teleport into a sealed room, you don't need a clear path.
Hermetic magic knows about this. As Heremetic magic knows that the world is a sphere orbited by the sun, and that the limit of the universe is the lunar sphere.
Magi can also easily observe the phenomena experimentally by Leaping large(ish) distances...like from one side of the Tribunal to the other.
The "Greeks" knew it, the Arabs knew it, the Egyptians knew it... And the Spanish, the vikings, the Celts etc etc...
It was "general knowledge" but not everyone was aware of it because they didnt need it and didnt travel enough to notice it on their own.
So in just about any area you can probably find people who knows about it even if its unknown if it was known to "all" people.
Im almost sure its discussed within the christian church as well.
So what? The trigger isnt "night", its "sunset".
Hmm, another variant... And rational as well... Getting problematic. :mrgreen:
Indeed. And sunset is the "disappearance of the sun below the horizon" (en.wikipedia.org/wiki/Sunset). You teleport, it disappears, voila: sunset.
What I'm trying to convey is that sunset is not the beautiful, slow "sinking" of the sun that many love to watch.
It's an instant defined as "before sunset, (at least a portion of) the sun is above the horizon, after sunset, it is not".
This matches the scientific definition as well as the linguistic rendering of the word both in English and in Latin.
As I already said, it's sunset, Occasus, not sunsetting, Occidens.
This has very little to do with anything - my point was that if the world was accepted as being flat in this age, then it IS flat in mythic europe. In a similar manner, if the mythic reality is that the sun sets at the same time everywhere, then it does so...
This also simplifies this discussion a lot, doesn't it;)
As has already been stated several times, no the claim of medieval belief in a flat world is a 18th century invention and a complete myth.
The only real "flat world belief(if it can be called that)" was on a philosophical level which had nothing to do with actually believing the physical earth to be flat.
Mythic Europe in 5th Edition is more or less based on Aristotle's physics.
Which include a round Earth, around which the moon, the sun, the moon, the planets and the "fixed" stars rotate.
I'd use sunrise and sunset at the location of spellcasting/item activation if I had munchkins among my players.
With a troupe unlikely to abuse it, I'd go for location of the spell at sunset. So if you blind an old man and teleport him to a different time zone, sunset at his location is relevant.
I don't think that I would be too bothered by munchkinism around this issue. Afterall, magi are munchkins. I think that if there was something to be exploited, then magi would try to exploit it, and as any exploit is based on in-game considerations of astronomy, movement of the spheres and so forth, it feels like the right kind of exploit --- it's not an exploit based solely on an accident of out-of-character game mechanics.
It is difficult to see an exploit that gives a magus much benefit, anyway.