^^^ How would I go about doing that? The game in question has IGT, but it is very weird. It is always faster than RT (by a lot, so it wouldn't just be loads), and I feel that some of the glitches for the game are situations where the game stops timing. I would really like to use both, because if users could submit IGT, I feel that it would encourage more people to run the game, but I also want to keep RT, at least for 1st place, because then I know that is the true time for absolutely everything done in the run, glitches included. Any advice? I am a Super Mod of the game if that matters.

You enable the in-game timer, that's it you have both. You can also select if default timer is in-game or real-time. (which one makes the placement in the leaderboard) but you still have both.

Sounds like a situation where you'd either select RTA or IGT as the timing standard for the game (like how Super Metroid eventually chose RTA) or a chance to split each category into two and develop different routes optimized to each.