An argument came at our table about the duration of the Shield spell, the crux of the matter being the interpretation of the words "until the start of your next turn".
From the Shield spell description (PHB, p. 275, emphasis mine):
Duration: 1 round […]
An invisible barrier of magical force appears and protects you.
Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from
magic missile.
Two cases were made :
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When cast using a reaction, the Shield spell lasts until the caster's turn in the initiative. Thus, if the spell is triggered by a monster attacking with an initiative of 20 and the caster acts at 19 (beginning it's turn), the spell will have a very short duration. On the other hand, if the monster triggers the spell at 20 and the caster's initiative is 1, the spell would last almost for the entire round, protecting the caster against every other monsters acting after the 1st.
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When cast using a reaction, the Shield spell lasts until the NEXT caster's turn, which means the spell triggered at 20 would last through the caster's turn (19) and then go on until the beginning of his NEXT turn (initiative 19, round 2). That creates a situation where the caster is protected against Monster 1, then all other monsters in the round, then again against Monster 1, since the caster's NEXT turn still hasn't begun when it acts in the second round.
As I argued that the same instance of the spell couldn't protect the caster from the same Monster two rounds in a row, my friend assured that my ruling (#1) would make the spell so weak that, considering the spell slot economy, nobody would ever want to use it.
As a sidenote, the stated duration of the spell (1 round) could be misleading, since it would imply that it MUST last an entire round (as it would if the spell was cast from a scroll or a magic item). Maybe the wording "up to 1 round" could have been more accurate?
Which reading of the spell is correct?
Best Answer
Your ruling is correct.
The effect of shield ends at the start of your next turn, which is the next turn you take. Your ruling #1 is correct.
There are many spells which have a listed duration of 1 round, 1 minute, 1 hour, or so on with a trigger listed in the text that causes the spell to end before that duration is reached. Although we might prefer the spells listed their duration as "up to 1 round," "up to 1 minute," or so on, the implication is indeed that that duration is a maximum limit. So the listing of a 1 round duration does not contradict the listing of an ending condition triggered by the start of your next turn.
Justifications for this ruling.
Each time a creature's turn ends, the next creature's turn begins. So your next turn is the occurrence of your creature acting following the last turn you took, whether or not a full round of turns has elapsed. This is the logical consequence of Jeremy Crawford's clarification on the flow of turns (see this tweet):
Without going into a needlessly formal proof using the well-ordering principle, Crawford's statement establishes that your next turn is defined by the ordering of the turns as they elapse, not by the quantity of turns that have elapsed.
Also, a stated design intent in 5e is that words should be read according to their idiomatic meanings. In English, "your next turn" means "your next turn," not "your next turn or after 1 round, whichever is sooner." The simplest interpretation is both intended and correct.
Why it is a good Level 1 spell.
Your friend's reasoning that shield is weak is fairly unsound, and ruling #2 is neither necessary nor supported by the rules.
It's not that you're losing future time on the duration of the spell if you cast it too late in the round before the start of your next turn. You are de facto protected from all attacks for a whole round between any one of your turns and your following turn by (A) the reaction casting of shield mid-round, by (B) the ongoing spell effect after that until your following turn, and by (C) the threat of what we might call "potential shield" before it was actually cast (since before you cast shield there were probably no attacks that targeted you anyway and for the entire round enemies may have chosen not to attack you due to knowledge that you possess the potential to cast the shield spell).
In my experience in games in which I have been a player or a DM, squishy casters like Wizards have used it at least once but usually twice per session. Now note that a +5 bonus to AC is more than what any mundane armor or rare magical armor can confer and it can often be relied upon for most of the attacks that target you for a whole encounter due to the spell slot and action economy. You even get to wait and see if an attack would hit you before you choose to use it.
Therefore, I assert that it is a plenty powerful and comparable Level 1 spell with good utility. It does not need to be fixed. Stick with your correct ruling.