To calculate the time of your run, you need to calculate the total frames between the first frame of the run, and last frame of the run (for all categories). LiveSplit timer is good for knowing how well you do during the run, but is not used at all during the actual timing of the run.

First frame of the run to count, should look like this:

That is the FIRST frame you see after the black screen. That frame happens 3 seconds after clicking on a level in the main menu.

Last frame of the run to count, is the FRAME BEFORE this one:

Which means, the run ends on the first frame when the BLYM is looking directly at you. Note that the frame above is NOT added to the total count.

Finally, you divide the total number of frames by 60 or 30 (depends on your video FPS rate), and get the final time.

How to time the run with this knowledge: Open your video with a video editing program which lets you see and move at exact frames. (I personally use ShotCut, which is a free open-source program. Other programs might work a bit differently)

Go to the first frame I mentioned and note the exact time of the frame, which should look like this: 00:00:05:19 (or 00:00:05.19) This format is (Hours):(Minutes):(seconds):(frames)

Got to the end frame I mentioned and note the exact time of it as well, for example: 00:02:07:21.

Now, you can either calculate the difference (in frames) between the two times to get the total time of the run, or cut the video between those two frames (so that it includes the first but don't include the second). You can see the total time after the cut, giving you the duration of the run.

If you calculated a total number of frames, divide it by 60 (or 30) to get the final time. If you look for the total run time in ShotCut, you will see for this example 2:02.02, just make sure to divide the remaining frames by 60 (or 30) as well.

(Those times were taken from the video of my world 1 glitchless run in 2:02:033)