When recording, usually people use livesplit. Sometimes people use sony vegas and add a timer on there. However when there's no timer on the video, I found it easiest to time it using this site:
You can use < and > to go frame by frame.
(The first frame being the frame the screen flashes white after clicking start, and the last frame being when the frame the final IGT appears in the box at the end.)
The reason real-time is used alongside IGT is because the timer pauses when you click off the game. RT is the most accurate time if you take that into account.
Also regarding that site, I found it accurate enough to use here but it only reads up to the hundredth, and not the thousandth decimal place.