The Dungeon Master's Guide provides optional rules for Feywild magic. One of these is Time Warp, where 1 day in the Feywild ranges from 1 minute to 1 year "everywhere else in the multiverse" (p. 50). It also states, "A wish spell can be used to remove the effect on up to ten creatures." (Ibid.)
If using the optional rules, when the Time Warp effect is removed do the creatures' ages change or do they move through time or something else entirely?
For example, in Faerûn, the party enters the Feywild and adventures for 1440 days (almost 4 years) and then returns.
- If 1 day in the Feywild = 1 minute everywhere else: The party returns 1 day after they left. Does wish reverse age them 1439 days or does it jump them forward in time 1439 days? If the former, are there experience point implications?
- If 1 day in the Feywild = 1 year everywhere else: The party returns almost 1444 years after they left. Does wish age the creatures 1440 years (effectively killing them) or do they jump back in time about 1436 years?
Best Answer
Wish removes it by making it not happen when you leave
The effect referred to is discovering that time has passed at a different rate while you were gone. Wish can be used to remove the effect of time passing at a different rate while in the Feywild — it doesn't remove the effect when it isn't affecting the creatures, after they've left the Feywild. Once they've left the Feywild, the effect is gone and can no longer be removed, and its consequences are permanent.
Age, dead loved ones, etc. are not the effect referred to, and aren't even effects in game terms. "Being in a later year than expected" is also not an 'effect' that could be removed (it's a lasting consequence of an effect already suffered and now gone), so wish won't time-travel someone in order to remove this non-effect.
Summary of what does happen
Wish can be cast to remove this effect from a group of creatures, so that they (and only they) are immune to the Time Warp of the Feywild when they leave. This immunity only applies if the spell has been cast before leaving. If wish is used this way, when they leave the Feywild, the time elapsed is the same as the time spent in the Feywild.
Any creature excluded from this casting of wish is not protected, and suffers any effects of Time Warp normally when they return. This means that casting the wish to include some people, but not others, will means that they return to their own plane at different times, regardless of whether they leave the Feywild together or separately. (Whether pulling such a stunt on a disliked fellow party member is considered good roleplaying or an unforgivable betrayal of the real-life player will depend on your group's social contract.)
Beware optional rules
Be careful relying on the RAW at all, though. Because Time Warp is an optional rule, the DM has no obligation to use it as written, and may choose to ignore it, modify it, or come up with something else entirely.