Is there enough time to Planar Bind a creature conjured by a 1-hour-duration spell?

Question

Many planar entity conjuration spells, such as Conjure Woodland Beings, last up to exactly an hour, at which point the creature vanishes:

A summoned creature disappears when it drops to 0 hit points or when the spell ends.

The Planar Binding spell normally takes exactly an hour to cast, but lasts a long time (24 hours by default, more if you upcast it):

With this spell, you attempt to bind a celestial, an elemental, a fey, or a fiend to your service. The creature must be within range for the entire casting of the spell. At the completion of the casting, the target must make a Charisma saving throw. On a failed save, it is bound to serve you for the duration. If the creature was summoned or created by another spell, that spell's duration is extended to match the duration of this spell.

Imagine the following scenario: Bob the Druid Readies a Conjure Woodland Beings for when Dylan the Wizard says "go". Dylan the Wizard says "go", and so, one or more DM-determined fey creatures appear as a result of the released Conjure Woodland Beings (let's say two quicklings), and right after saying "go", Dylan starts casting Planar Binding on one of the feys (here, a quickling). Both spells were cast at almost the same time (same turn), and both last an hour. What I wonder here is whether Dylan will have enough time to Bind the creature before it vanishes, or whether he'll be "one second short".

Is there enough time to Planar Bind a creature conjured by a spell with a duration of 1 hour? (Discarding techniques to reduce Planar Binding's casting time, such as Wishing it, and techniques to extend the base duration of the Conjuration spell, such as the Extended Metamagic option.)

Answer 1

Maybe, but it needs a particular setup, buy-in from the GM, some suspension of disbelief, a willingness to outright abuse the rules, and a, um, flexible definition of when time passes during a combat round

Essentially, we're going to abuse the mechanics here in a way that doesn't make sense if you try to explain it outside of game mechanics. In particular, we're going to abuse the timing of the combat round.

Setup

So, you can't just say that you're casting Planar Binding at the very moment that the conjuration is finished, because that will almost certainly have just the slightest bit of delay (and thus creating that "one second short" situation you mentioned). Unless of course your GM just says "yeah, that works", in which case you don't need this. At the very least, I know that I wouldn't give that to you.

So instead, arrange to be in combat as the conjuration finishes (how you do that is up to you), and make sure you can stay in combat for a full hour. In your first turn immediately after the conjuration is finished, you begin casting Planar Binding.

Then, you need to stay in combat for 600 turns and not break concentration, and now your spell, somehow, finishes just before the summoning wears off!

But how does this work?

Well, a battle round represents six seconds of simultaneous combat. However, we're abusing that simultaneous part with the use of turn order. Even though everything in a battle round technically happens at the same time, everything does end up having a particular order mechanically.

So, to pull from your example, your druid finishes the conjuration and two woodland creatures appear. Your wizard, during their immediate next turn, begins casting this spell.

At this point, you're now in the weird physics- and time-breaking situation in which your wizard has already spent six seconds casting Planar Binding on a creature that the wizard summoned zero seconds ago! From here on out, your wizard, at least from a purely mechanical perspective, gets to experience those six seconds just before your druid does! And since your druid's spell wears off one hour after the end of the druid's turn, that means it wears off only at the druid's 600th turn!

Incidentally, Planar Binding finishes casting during your 600th turn. And, wouldn't you know, you just happened to get that one turn head start, so now your 600th turn will happen just before the druid's 600th turn! And thus, you will finish Planar Binding just before the conjuration wears off!

Of course, this does mean you have to stay in combat for 600 turns. I don't know how you'll manage that, but this is very important. The moment you fall out of combat, combat rules stop applying, and common sense makes this once again impossible to do. This will not be easy to do.

Results may vary

This method of course does depend heavily on what your GM is willing to tolerate, and what kind of game they want. A GM who likes more gritty or realistic games would probably not allow this, since it actually makes zero sense outside of counting combat rounds. These GMs will follow the idea that since you started casting Planar Binding after the conjuration was finished, then it's impossible to finish before it wears off regardless of how the rules can be twisted around.

On the other hand, a GM who likes following rules to the letter, with little or no regard to those rules' implications regarding the game world's physics, would probably allow this interpretation - you just have to convince them that this is indeed how the rules work.

There's also the chance that you might impress your GM with the sheer audacity of this plan, enough for them to say "Sure, if you can pull it off and stay in combat for 600 turns, I'll give it to you."

Why not just do this outside of combat?

Because Planar Binding requires a target, and the target you have in mind doesn't appear until after the conjuration spell is completed. Thus, you cannot begin casting until after the conjuration's duration has begun.

About the weirdness of the passage of time, and the assumptions I made

This answer relies on an assumption that the six second time unit only applies during combat, and that it cannot be taken advantage of unless combat rules are active. In other words, I assume that common sense rulings are in play if the rules give no indication otherwise.

If we assume instead that the six second unit of time can apply outside of combat, then this answer becomes unnecessarily complicated, and all one has to do is say "I begin casting the moment the spell is finished," and then maintain concentration.

Answer 2

Yes, there is enough time for Planar Binding.

First, we know that the Planar Binding, PHB 265, is cast after the conjuration. An in-depth answer on order is available in this thread. This is important because the Planar Binding requires a target.

Second, we know that a round is a unit of time as explained in Time (PHB 181); compare to The Order of Combat and the Combat Step by Step (both PHB 189). There is only one round at a time - rounds do not overlap, and rounds end after all turns of the round have passed.

Keep in mind that only Duration spells occur simultaneously. They conclude at the end of rounds (instead of turns), so this is one of the few instances where order isn't determined by Initiative or trigger.

This perception of time is explicit for Duration spells (PHB 203):

A spell's duration is the length of time the spell persists. A duration can be expressed in rounds, minutes, hours, or even years. Some spells specify that their effects last until the spells are dispelled or destroyed.

A duration cannot be expressed in a shorter interval than rounds. This is also true out of combat, so Duration spells always start in a round. Both of these spells are not Instantaneous (PHB 203). Instantaneous spells happen on turns (or during another character's turn).

So knowing these facts we can safely assume that both spells resolve in the same round, first your conjuration, then your binding but they still occur in the same unit of time.

The duration of Conjure Woodland Beings (PHB 226), is an hour which is also the time required for the Planar Binding, the binding takes effect and extends the spell as read in the description to 24 hours and longer at higher levels.

As both of these spells are spells with duration, it makes it certain that there is enough time, and that there is no abusing, bending, or setup of rules or even a battle required as MrSpudtastic implicates in their answer.

Answer 3

To add to the answers given already, see Jeremy Crawford's tweet on essentially the same situation:

Does planar binding work with conjure elemental or does the Elemental disappear 6 seconds too early?

As DM, I'd allow you to pair conj. elemental with planar binding if the latter starts when the monster appears.

Even though tweets are no longer considered official rulings, it supports the idea that allowing players to use these spells in conjunction wouldn't violate the game designers' intent for them or be too overpowered.

Answer 4

Who's on first?

Tl:dr this should work without issue out of combat with the proof below.

If you don't have a DM who will just hand-wave the closeness of time to allow this (or he wants to hand-wave it to not allow), then you need to look very closely at the duration specifics.

While the time for both is 1 hour and rounds are tracked in concurrent 6 second intervals for the participants, things do get adjudicated by the turn.

This can be seen in this question about when duration spells end.

If we do go with the answer that the duration will end before the following participant's next turn starts, then we have a method to adjudicate this question.

Conjurer is 1st, Binder is 2nd: Binding Fails

In this case, we are likely 1 turn into the rotation. THis has allowed the Binder to Ready their action to cast planar binding in the same starting round that the conjurer has brought for their woodland beings.

At the end of hour turn, the following should happen:

1. Right before the Conjurer's turn, the animals disappear at the end of their spell.
2. The Binder has not fully completed their spell (it'll finish at the start of the next round's turn) and therefore the spell fails because the conjuired animals are gone.

Binder 1st, Conjurer 2nd : Binding succeeds

In this case, the Binder's readied action goes off later in the round when the Conjurer summons their animals.

At the end of hour turn, the following should happen:

1. The round begins with the Binder completed their spell and the animals present are bound.

2. The Conjurer's turn comes with the animals not having disappeared because they are now bound by Binder.

Since one works, it'll work out of combat

Since there is a path to success, it'd be trivial to say that the casters involved know this and would perform it in the right order.

As there is the clear path, then if out of combat, this will absolutely work.

Answer 5

Target Need to be There the Whole Time?

As I was thinking about this more, something popped into my head. The whole problem is predicated on the idea that you can't start casting until the target is there. So, I looked up targets, and found this:

A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below).

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature's thoughts, typically goes unnoticed, unless a spell says otherwise.

Which doesn't say you can't start casting a spell until the target is present, it could be possible (no rules against this idea at least) that the target only has to be present when the spell finishes.

But, what other support could exist for such an argument? Well, Readied spells.

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs.

When you Ready a Spell Casting action, you start casting and hold the release of the spell for the trigger. If you don't end up hitting the trigger, you still lose the spell slot. When you ready a spell for a character that has blink or invisible to reappear, you're actually starting to cast the spell and then releasing the spells energy at the target. If there is no target, the spell fizzles and you still lose the slot.

Seemingly Simple Case: Ready an Action

Let's start with the simple case. The person casting planar binding readies the action for the trigger of the conjured being(s) to appear. Readied actions start instantly upon the trigger, and thus the vanishing and the finishing of binding happen at exactly the same time. It is technically a coin toss, but I think most sensible DM's would resolve that instead of disappearing, the conjured beings stay around.

However

Except this doesn't work. You can only ready a spell that has a casting time of one action.

To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration.

Out of Battle

This should work fine, just start casting the binding spell before the conjuration.

Battle Rounds

The argument over time is caused by the simulationist version of time in D&D. A round is 6 seconds, regardless of the number of creatures in the battle, because it is supposed to simulate all the actions happening at once. If that is true, even in rounds, the binding spell should hold if it was started in the same round as conjuration.

But, we know that order of rounds matters quite a bit because of things like reactions recharging, different spells lasting until at the start or end of your next turn, and of course if you die on someone else's turn you don't still get part of a turn to act on... So rounds are both happening at the same time and not. In this case, your hour is a few seconds late and beings already poofed and the binding shouldn't take effect. And, I can see an argument for battle making you miss the window by a few seconds. Or, you can take advantage of this, and start casting planar binding before the other player casts the conjuration spell, and it should work because of the above section on targets.

Rule of Cool -- Risk v. Reward

Planar Binding in combat is dangerous. You're gambling 600 rounds where you can't take actions, a fifth level spell slot and that the conjured beings will live those 600 rounds, that you and the conjurer won't lose concentration and drop the casting (or lose control of the beings). That is one heck of a gamble, and as a DM having it all just fall apart by having the beings just vanish a second before you finish would be a terrible way to repay the risk and gamble.

Talk to Your DM

Before you mean to try it, have a conversation with the DM. Tell him your worried about the risks, and ask them how they would rule. If they don't tell you how they would rule, then try it in game on a downtime/low risk session, and see what happens.

Keep in mind that Jeremy Crawford would let you when you are taking to your DM about it:

As DM, I'd allow you to pair conj. elemental with planar binding if the latter starts when the monster appears.