Create time lapse video from other video

To take every 15th frame, use the select filter

ffmpeg -i in.mp4 -vf select="not(mod(n\,15))",setpts=N/FRAME_RATE/TB out.mp4

Another method is to use the framestep filter

ffmpeg -i in.mp4 -vf framestep=15,setpts=N/FRAME_RATE/TB out.mp4