Taking a few minutes to copy the spell and adjust the copy to account for planetary drift was sloppy, without any extra failsafes, but Celestia believed it would work. On a hunch of what kept Twilight from returning, she created a third copy of the spell, that also removed the vocal requirement of the spell, instead triggering a return when the caster opened their wings.
‘Here I come, Twilight.’
Lighting her horn, she quickly cast the modified spell, letting her power course through the matrix. Nodding, she finalized the spell and a small vortex took her.
Invisibly sliding into existence a meter above the ground, she appeared seconds before Twilight cast her version.
Unwilling to interrupt the cast, she latched onto Twilight’s signature. Her magic copied the spell and analyzed it to set the same destination. She then cast the non vocal version of the spell, following Twilight through time.
Flashing back into existence, she instantly felt the air leave her lungs. Mere meters away, Twilight thrashed as she panicked. Lighting her horn, she pulled Twilight to herself. Wrapping her legs around Twilight, she tapped into the last of her power and triggered a return to where she originally landed.
Landing back in Twilight’s past-chambers mere seconds after they had departed, she gathered her wits.
‘First, stabilize. Make sure she starts breathing again.’ Instinctually tapping into her connection with the sun to recharge her reserves slowly, she set about seeing to Twilight.
Twilight was absolutely freezing. Checking herself over, Celestia had a layer of frozen water sweat melting off her coat. ‘Warm her up after.’
Laying down next to Twilight, she watched, looking for signs of life. The mare wasn’t breathing, but her heart still beat faintly.
Quickly, Celestia set about performing CPR. ‘I can’t lose you, Twilight. You just joined us!’
Twilight’s heartbeat picked up as Celestia pushed several lungfuls of air into Twilight. She set about keeping a steady stream of air in and out of Twilight’s lungs for several minutes, until Twilight picked it up on her own.
Sputtering softly, Twilight returned to consciousness several minutes later. She still felt frozen cold, but was wrapped in warmth. Blinking softly, she couldn’t quite see past the frozen tears over her eyes. Twilight groaned softly, “I’m… alive?” The warmth shifted slightly, before laying something over her eyes.
“Yes. You are.”
Twilight froze stock still at the voice. “Celestia?”
“Mhmm.”
She tried to shift slightly toward Celestia's voice, but a hoof against her side held her still. “How?”
“I did what you would’ve if it was your friend. I did some quick math.”
“The failsafe didn’t…” A large warm blanket slid over Twilight's body, doing wonders to soak away the cold, frigid feeling across her body.
“Shhh. I know.”
“I had to make two modified copies of that spell to get to you.” The cloth lifted away and Twilight saw Celestia cuddled up to her, doing her best to warm Twilight up.
“Two?”
“Mhmm.” Celestia nodded. “One to fix what was wrong, one to make sure I could cast it in the vacuum of space.”
Twilight’s horn lit, only for Celestia to knock a hoof against it softly, the purple glow fading away. “None of that. Rest. We’ll be returning to the present shortly, once you’re not in immediate danger.”
“We’re in the past?”
Celestia nodded softly, the corners of her vision clouding. “Kind of. I had to come back to when you cast the spell originally and follow you that way, to know exactly where you went. Anything else would’ve been a guess, and I wasn’t taking that chance.”
“I… I’m sorry, Celestia.” Twilight moved to continue, but the same hoof silenced her.
Tears in the corners of Celestia’s eyes finally fell, streaking down her face to drip on the floor. “No. Don’t be sorry. Luna is already sorry for destroying my balcony. I just want to know you’ll be alright, okay? I can’t lose you, not again.”
Twilight wisely stayed quiet, nestling into Celestia’s warmth more and closing her eyes. Celestia responded by curling around Twilight tighter.
Several minutes later, Twilight spoke up. “Celestia?”
Celestia lifted her head. “Yes?”
Twilight's eyes were still closed and her ears pinned back. “I… I’m sorry for all this. I was hoping to surprise you.”
“I told you already, don’t be sorry. I’m just glad I was able to save you.”
She lifted her head and gazed into Celestia's eyes. “Do… do you want to know why I created this spell?”
“Only if you want to tell me.”
Twilight was visibly uncomfortable, shifting slightly under Celestia's wing. “I was… I was hoping to go back to the past and ask your parents’ permission to court you, formally.”
“Oh?”
She opened her eyes and looked into Celestia's lilac orbs. “I… I love you, Celestia. When your sun comes up in the morning, all I can think of is how happy you make me.” She started to sit up. “I… even if you never wanted to date me, I would be fine being your fmff!”
Twilight was silenced again, but far more pleasantly. Celestia’s lips met hers softly. Several seconds later, Celestia pulled back. “I don’t care if they had said yes or no, Twilight. I will very happily accept your offer to court me.”
Twilight’s eyes widened as Celestia teared up again. “Just… please… take more care in your spellcraft. I’m not sure if I can rescue you next time.”
“I will, Celestia. I swear it.” Twilight whispered with the most sincerity she could muster. Reaching a hoof out, she drew Celestia’s head back down, and licked her tears away. “I’m sorry I worried you so much.”
Celestia, shocked at Twilight’s forwardness, buried her nose into Twilight’s cold neck. “L-love, please don’t do it again. You have been very dear to me for quite some time, and I couldn’t bear losing you now…”
After what seemed an hour passed, Twilight was back to normal. She no longer felt cold, nor did she shiver against Celestia.
“Twilight, are you ready to go back?”
Twilight stood and shook her limbs out, testing to make sure each felt fine. “I believe so.”
Celestia’s telekinesis reached out and pulled Twilight against her barrel, a hoof holding her in place. “Then let’s go.”
Celestia’s horn lit as Twilight pulled Celestia’s muzzle down to plant a soft kiss on her marefriend’s lips. With the sound of a soft breeze, both ponies vanished.