Using video recorded by iPad how can I calculate the exact time duration between 2 frames? I shot a video with an iPad recently for a school project. 

My teacher told me the iPad shoots at 30 fps, but opening the video afterwards VLC tells me the frame rate is 29.970030.

By watching the video and stepping through the frames, how can I calculate the exact time duration between 2 frames?

E.g. what's the time difference between frame 100 and frame 130? Is it 1 second (assuming 30 fps) or slightly more/less?
iOS' native camera app (iPhone/iPad) does not have a fixed frame rate.

I attached the details for a short clip I recorded. It varies between 28.571 fps and 30.000 fps



VLC displays a frame rate of 29.973708 fps for the same clip.