Does Planar Binding extend the duration of "another spell" after the completion of casting Planar Binding or before the completion?

Question

Planar Binding (PHB 265) has this clause:

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.

The last sentence clause is unclear. Does Planar Binding extend the duration of the "another spell" after the completion of casting Planar Binding or before the completion of casting Planar Binding or both?

Regarding most spells, it is not important whether Planar Binding extends the duration of "another spell" after the completion of casting Planar Binding or before the completion because all spells that summon creatures have a Duration (PHB 203) of up to an hour. This duration of up to an hour seems to be by design, and Planar Binding works regardless of the last sentence due to the durations rules assuming that it is correctly explained here.

There are two cases that I can think of which make the distinction of before or after completion critical, one in this thread that answers:

What happens if I cast Planar Binding on a creature who was brought by a spell that has ended?

The second case is: what if I allowed a homebrew spell such as SeriousBri's Magnificent Summoning? This spell has a duration of 1 minute that is far less than an hour, would Planar Binding count the "another spell" in this case as SeriousBri's Magnificent Summoning, thus extending its duration to match the duration of Planar Binding under the assumption that Planar Binding's effect of extending the duration of the "another spell" happens before the completion of casting Planar Binding?

Accordingly is there generally enough time for Planar Binding a creature conjured or summoned by a spell regardless of that spell's duration, under the assumption that Planar Binding's effect of extending the duration of the "another spell" happens before the completion of casting Planar Binding and that the effect of extending the "another spell" also persists for spells that have already ended?

Answer 1

After the casting of planar binding only

Spells have no effects before they are cast. Casting a spell is a necessary prerequisite for the effects taking effect.

A spell is a discrete magical effect, a single shaping of the magical energies that suffuse the multiverse into a specific, limited expression. In casting a spell, a character carefully plucks at the invisible strands of raw magic suffusing the world, pins them in place in a particular pattern, sets them vibrating in a specific way, and then releases them to unleash the desired effect--in most cases, all in the span of seconds.

Similarly the PHB also describes how to cast a fireball spell:

Clad in the silver robes that denote her station, an elf closes her eyes to shut out the distractions of the battlefield and begins her quiet chant. Fingers weaving in front of her, she completes her spell and launches a tiny bead of fire toward the enemy ranks, where it erupts into a conflagration that engulfs the soldiers.

Note how there are absolutely no effects of the fireball before the casting ends.

Without completing the casting of a spell, you can't have any effects. There is simply no way for that to work. The rules are built on this assumption.

If one of planar binding's effects was meant to be an exception to this it would be extremely odd, but also it would explicitly say so. But it does not.

As such, the "another spell's" duration is only extended after the completion of the planar binding casting.