I just started speedrunning doom for xbox one and I’m wondering if I have to set up and use a timer, or can I just use the in-game timer.

I'm pretty sure you can use either one

Okay thanks

For an IL you can use IGT, but if you're doing entire chapters you need a time as in between stages counts in that category

For IL's the in game timer is used. For full game/episode runs, use RTA with either livesplit or whatever other timing method you choose. Timer on screen is not required since all mods retime every run.