Gerion of Mercadia
First Post
Why not? The duration is not concentration, permanent, or instantaneous, so the feat is not prohibited from applying.
Right, the actual duration is "null"
Extend Spell makes the spell last twice as long, which means the spell effect begins and ends faster than they can so much as blink twice.
You're sped up for twice as long as you would be with a non-Extended Time Stop.
Twice "null" is still "null" in a similar fashion that 2 x 0 = 0.
Legally, you could apply extend spell - but you still can't double nothing.