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 :
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.
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?