Do you have to use an external timer or can you just time game time using an editing software or something

you can use an editing software like virtualdub to time your run after it is finished.

if by game time you mean ingame time, I don't know. I don't know this game.

No i mean like a mod when he/she goes to review my run, they can time when i start playing in the video to when i finish in the video. so say i started actually playing at 0:30 seconds in the video and i beat the game at 7:45 and ended the video at 8:15

That means my time would be 7m 15s

yes time before and after the run doesn't count

Why can't you time it when you post it ? Just tell the time of your run from start (When you get the control of your character) to credit clear on Stage 8 in your submission and a mod will verify this.