We know what real time is, as opposed to in game time, right? What do we do when an in game timer is made by someone in the community? If it works, do we implement it? Can we re-time the already existing runs? I know this varies from game to game, but I want to hear your opinions and thoughts.

What do you mean in-game timer that was added by the community? As in a modification for the game? Or do you mean a timer that doesn't count load times? (In that case it's not really IGT, but time without loads that you can get with an autosplitter in Livesplit).

Depends on the nature of the change between the timers, it might be easy or very hard to retime existing runs. And this indeed varies from game to game, you as a community for the game should know better than everyone else about what you need.

I guess my point is that more information is needed about this new timer.

Yes, it entirely depends on how the community want to time the game.

For instance, Sonic The Hedgehog is perhaps my favourite example of this. The leaderboards go so far as to list BOTH types of time; that is, either pure IGT or the frame-by-frame exclusion of time bonus from RTA. The moderators even have a helpful little section on the rules explaining both, how to self-time AND a note that if you are not comfortable with either way, that the mods themselves will retime it for you anyway.

Obviously this is easier to do on a short game. If you're talking about re-timing a massive leaderboard of 7 hour long RPG runs then the community might want to think long and hard as to whether it is worth it, or whether sticking with your current method is more appropriate.

In this case, with "in game timer" I mean a timer mod that is added to the original game. I feel like this will make things a bit more complicated for people that just wanna do runs and not mess with the game before running.

Well, can you elaborate further? Why did someone made a mod for a timer? How does that timer work? Is there any major difference between this new in-game time and a regular RTA time?

Allowing custom modifications to a game is an incredibly slippery slope and is best avoided if possible by requiring all runs to be done on unmodified versions of the game. Allowing mods opens up a rabbit hole because how can you verify someone didn't install another mod that say removes RNG along with this timer mod? Or what if someone creates a mod to remove all RNG from your game, will you allow that one too?

It’s just dangerous practice to do that in my opinion, and if someone has the developmental skills to modify the game to add a timer, I would implore them to create an autosplitter for the game instead to remove the loads because then that can be controlled easier.