How to calculate end video time?

Question

fmpeg

Good afternoon,
I would like to split the video into keyframes and collect the necessary pieces back.
I have data about keyframes, of the form:

["0.166834","0.333667","5.338672","10.343677","11.886887"...]

I'm trying to guess the exact time of a future video by adding and subtracting the keyframe data, but the final video is always off by a few seconds. Nothing visually noticeable.
Do you have any guesses why this is happening? I don't encode video, I just break it into keyframes and put it back together with the same codec.