Wait, I got an idea.
What if... right before the day of Walpurgisnacht... we take Mitakihara, and move it west? As long as we keep moving it just quickly enough, it'll never be the day of Walpurgisnacht in Mitakihara. Then once we've done this for 24 hours, we just end up with Mitakihara back in its place, having skipped the day of Walpurgisnacht's attack.
Walpurgisnacht will be so confused.
"I. Can't. Believe. This. Worked."
*Holds Homura*
"You OK there, friend?"
"Give me a minute. Or sixty. Thousand."
"As long as you need."
'Hey Mami, Sabrina, you won't believe what happened.'
'Kazumi? What happened?'
'We had a big fucking Witch attack in Asunaro!'
'WHAT!'
'And we kicked its ass! Niko says the Grief Seed's shiny and wants you to do some spooky Grief magic on it. That's why I'm calling, actually.'
'... You girls are all OK.'
'Yes.'
'Good. That is very good.'
'Anyway, give Niko a call. Not now, though. Later, she's in the zone right now, you know?'
'Yes.'
'Later.'
'Later.'
"..."
"What is it?"
"... Nothing. It was nothing, Homura. Don't worry about it."
"..."
'Don't ever-'
'-tell Homura, I know.'
'I love you.'
I was going to say that the date line is there to ruin everyone's fun, but then you had it attack Asunaro and...