You get a bunch of elapsed time back, so you gain days. Not sure how many, but I think a fair bit.

So is that like drinking elixir or cowplant juice or do they get extra days added to their lifespan? Is this cashed in during the age group they were in during the wish or is it tacked on to their elder stage (if they're not in elder stage yet)?

Elixir adds days to lifespan, it just doesn't show.I had someone drink a full container of Elixir with only 3 days gone from lifespan (She had 28 other containers at that point, so I was clearing some inventory space)

I thought she had glitched when 6:00 rolled around and she didn't age. But when I checked with Inge's "Days Left" shrub, she had 39 days left, which would have been right. Her bar still only showed 29, though.