In this one Spike and Buffy are fighting a demon. The demon has the power to open portals in which it can transport their enimies for a specific set of time. In this case they begin to fight it and the demon opens a portal which Buffy and Spike fall into and they are then transported to a beach. I do believe this takes place after "The Gift" so Buffy has been ressurected, but I don't remember if it is before or after her "relationship with Spike. ut I could be wrong on both (sorry). The rest of the story takes place with them on the beach, it was a porn with semi plot story. After i think 7 days the portal reopens and they end up back in Sunnydale. Only to reconfront the demon and get sent back to the same beach.

TIA

I don't think there was a portal in this one but it sounds like what you were looking for. Buffy puts on a cream left by the trio that sends her to the beach and Spike follows soon after. He can be in the sun there. I read this on the Bloodshedverse.

Instant Sun by Verity Watson Takes place in Season Six, after All the Way, which means it is before Buffy and Spike kiss, but after she confesses to Spike that she was in heaven. Only no Giles on hand, sorry. Written because I've been dreaming of tropical islands in the midst of winter. Originally posted at BS Central. ***Winner of Judges Choice award, LLGA Round 19***

Melts, was this the story you were looking for?

It sounds like 400 days by passionfish