Screw those people for trying to monetize something you’re expecting to use for free. Your phone and computer have the capability to do timers. Why are you googling a timer with this attitude in the first place?
Source? At least on macOS, i'm not aware of any built in timer or stopwatch.
sleep 300; tput bel
timer() { sleep $1 ; i=$2; until [ $i -eq 0 ]; do tput bel; sleep 1; i=$((i-1)); done }
timer 300 10