Spirit Guardians, Dash, and Movement Speed Question 5E (2014)

I’m looking for some clarification on how Dash interacts with Spirit Guardians.

5e Defines Dash as:

"When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers. With a speed of 30 feet, for example, you can move up to 60 feet on your turn if you dash.
Any increase or decrease to your speed changes this additional movement by the same amount. If your speed of 30 feet is reduced to 15 feet, for instance, you can move up to 30 feet this turn if you dash."

Suppose a creature has a 30 ft. speed and takes the Dash action, giving it 60 ft. of movement for the turn. The creature begins its movement outside of Spirit Guardians and then enters the spell’s area during that movement.

Spirit Guardians says that an affected creature’s speed is halved while in the area.

How should the creature’s remaining movement be calculated when it enters the aura?

For example, if the creature moves 10 ft. before entering Spirit Guardians, does its movement allowance become 30 ft. total for that turn (15 ft. from its halved speed + 15 ft. from Dash), leaving it with 20 ft. of movement remaining?

Or, because the creature already took the Dash action while it had a 30 ft. speed, does it retain the original 30 ft. of additional movement from Dash, resulting in a different amount of remaining movement?

More generally, when a creature’s speed changes after it has already taken the Dash action, does the amount of additional movement granted by Dash change along with its current speed?

I am under the impression that the movement from dash would also be decreased by the same amount that speed was.

In any case, looking for some clarification. Thanks for the help!

Interesting. I've just realized the "Any increase or decrease to your speed changes this additional movement by the same amount" line is not in the Dash description for 5.5e, only the example.

It was useful, IMHO.

EDIT: it's also interesting, in the chapter about "odd spells", the spell doesn't use Difficult Terrain instead of halving Speed as other spells (e.g. 5.5e Conjure Minor Elementals). Again, IMHO, it would be easier to rule on.

Quote from spamandtuna >>

However, Spirit Guardians does say the speed is only halved while in the area. If the creature has enough movement to leave the area, their speed immediately returns to normal, letting them move the remaining full distance.

This is the part that seems a bit counterintuitive and probably unintended.  Even without considering dash, if your original speed is 30 and is reduced to 15 while inside Spirit Guardians and you use 10 feet of movement to exit that area then your speed immediately increases back to 30 and you can then move 20 more feet -- so it's as if you were never slowed down at all by being in the area.