I was investigating a litle,
If it was a debian based system you could run something like this
curl http://localhost/command/?cmd=stop | at date +"%T" -d '+20 minutes'
Since we are on Arch, I can't find the "at" command
I can't help more, but I'm shure thats a lot of people on this forum will have a solution for this.
After that is solved, it's a question of UI to make a screan to select the off set time and a litle php script to run a shel command