Someone

on *:TEXT:!start:#:{
  var %nickz $nick
  set $+(%,stopwatch.start.,%nickz) $ctime
  msg $chan Stopwatch started.
}
on *:TEXT:!stop:#:{
  set $+(%,dur.,$nick) $duration($calc($ctime - $($+(%,stopwatch.start.,$nick),2)))
  msg $chan Stopwatch stopped after $($+(%,dur.,$nick),2)
  unset $+(%,dur.,$nick)
  unset $+(%,stopwatch.start.,$nick)
}