I mean, you can just see the real time on the video, right? Can I just use the in-game timer?

Yes, timer is not required. It can be timed manually. IGT is not enough.

Furthermore, if you read the thread below yours, Ewil explains why this is the case

i know that this is probably not the place to post this but im not sure where i would because im new to this site, i tried to submit a run under the monday category and it was rejected with the message: Rejected: There needs to be a visible timer on the screen Ive done some reading and i do understand the whole igt not being counted because of saves/loads and deaths But the one and only current ranking on monday or any single day for that matter has the same format of video i had, that is not showing a rta timer but just in game and he made it onto the board with no issues so im wondering what im doing wrong and how i could fix it i was looking to do the single day runs until i get enough practice in to do full game runs or am i just missing a post that clearly states this

It should not have been rejected. Do you know which mod rejected it?