Are there rules and/or lore in any edition of D&D for managing time-travel paradoxes?

Question

Background & motivation

On this question about weather prediction, user dsollen commented that someone could potentially use Druidcraft to predict the future, using the following steps:

• Make a plan with a chance of success
• Plan, in addition, to use Control weather to create an otherwise unlikely weather phenomena if the plan succeeds.
• Use Druidcraft to predict the weather within the 24 hours prior to the event, effectively determining whether or not the plan succeeds based on the presence of your chosen weather pattern in Druidcraft's prediction

Whether or not the above is actually possible is out-of-scope for this question (That has its own question here). It just serves to show that rules interactions in 5e could be used to create time-travel paradoxes (in this instance, because reliable information is being sent backwards through time, and future events might change based on that information).

The Question

Are there any rules, for any edition of D&D, which give guidance on how to manage time travel paradoxes? In the absence of such rules (or, where relevant, in addition to them), are there any canonical pieces of lore which inform us of how time-travel paradoxes might resolve or be resolved?

Answer 1

Try AD&D 2e's obscure product Chronomancer.

This product (which is long out-of-print but purchasable in PDF form) includes rules for time manipulation, paradoxes, etc. It includes spells explicitly designed to cause paradoxes, such as the 3rd-level spell minor paradox, the more powerful 6th-level spell paradox, and the immensely powerful 9th-level spell major paradox. Chapter 6 is all about the consequences of using such magic and "guidelines" for DMs "to solve any problems that might crop up." (See p. 64). Chapter 3 also lays out ideas for time-traveling adventures and some of the player-facing difficulties that result from altering history.

As for lore, the only setting-specific lore on time travel of which I am aware comes from Forgotten Realms, and is detailed in this Q&A. Chronomancer, meanwhile, contains setting-agnostic lore involving "Temporal Prime," posited as "a pseudo-reality that permeates every plane of existence among the alternate realities," which the authors suggest was previously mis-described in other D&D products as a "Demi-Plane of Time." (See p. 4).

Finally, for the sake of completeness, in 3e/3.5e there were a few less-helpful, less-detailed, and in some cases less-official references to time-related gameplay:

• The 3e Manual of the Planes presents a "variant plane" called the "Temporal Energy Plane." Its description mentions time loops, but not time travel or paradoxes per se. (See p. 208-210.)
• An article on Wizards of the Coast's website entitled "Perilous Gateways: Portals in Time" sets out a teleport through time spell that provides a mechanism for time travel. The article goes on to warn the reader about paradoxes and offers the hand-wavey suggestion that "it is simplest to use the rule that changes in time are minor and somehow time smooths them out."
• Dragon Magazine #350 presented an article on "Chronomancy," which included the statement that "Currently, no spell, artifact, or creature in D&D can time travel over a span of years." The time-related mechanics presented in the article do not permit time travel per se. The article suggests that time travel might be "the purview of deities of travel and time[.]" (See p. 76-79.)
• Dragon Magazine #359 included a write-up on a monster called a "time dragon," the Great Wyrm form of which has a Time Apotheosis ability that permits it to travel backward and forward through time more or less arbitrarily. The material does not engage the possibility of paradoxes.
• There were a couple of psionic powers, such as the 9th-level power time regression (Expanded Psionics Handbook p. 138) and the more limited 3rd-level power forced dream (Magic of Eberron p. 104) that allowed the user to move backward in time by very small increments, which theoretically might be capable of creating paradoxes. None of that material explicitly addressed how such paradoxes might be handled.

Answer 2

The wish spell allows (very limited) alterations to the past in most editions

One of the the options for Wish in D&D 5e is (emphasis added):

You undo a single recent event by forcing a reroll of any roll made within the last round (including your last turn). Reality reshapes itself to accommodate the new result. For example, a wish spell could undo an opponent's successful save, a foe's critical hit, or a friend's failed save. You can force the reroll to be made with advantage or disadvantage, and you can choose whether to use the reroll or the original roll.

The first boldface phrase indicates that the spell literally changes the past by altering a "recent event". It's not quite time travel, but it still has the potential to create a paradox. The 2nd boldface phrase is all the direction given to the DM for resolving any such paradoxes that arise from altering the past event. (The 3.5e version of wish contains the exact same phrases.) Obviously this guidance leaves things almost entirely to the DM's discretion. However, if you find it useful, you could apply this guidance to time paradoxes caused by other means as well, though obviously doing so becomes substantially more difficult if the time span involved is longer than one round.

The OD&D limited wish spell (described here) also mentions altering the past, but as far as I can tell doesn't provide any detail or guidance on how the DM should implement it.

So unfortunately, while wish technically presents an official mechanic for altering the past, it provides minimal guidance for how the DM should actually adjudicate this mechanic.

Answer 3

Check out Doctors and Daleks

Doctors and Daleks is a product officially licensed by the BBC to allow playing D&D 5E in the Doctor Who universe. As far as I understand it isn't officially promoted by WotC and instead uses the OGL to make use of the SRD. Regardless of that, as you might think for a game set in a world with time-traveling policeboxes, it has extensive rules about time traveling in its Player's Guide, including a segment about paradoxes. So if you want some guidance on how to handle time paradoxes within the 5E ruleset, this might be a good source.