What stopwatch timer app uses keyboard shortcuts on macOS?
A low-tech solution is to open a terminal window, run time cat, and then stop by pressing control-c:

$ time cat
^C
4.487
$


Two ways to show the number of seconds since the last run:

f=${TMPDIR}stopwatch;date +%s&gt;&gt;$f;tail -n2 $f|awk '{l=$0;getline;print $0-l}'
now=$(date +%s);echo $((now-prev));prev=$now