To capture desktop as time lapse style is similar to the regular recording. We just need to change the input frame rate to a very low number, and set the output timebase, pts, and frame rate to what we expect. The settb = 1/(desired-frame-rate/input-frame-rate).
ffmpeg -f x11grab -r 1 -s 1920x1080 -i :0.0 -f pulse -ac 2 -i default -c:v mpeg2video -q:v 1 -vf settb=1/30,setpts=N/(30*TB),fps=fps=30 -an output.mkv