When measuring elapsed times, instead of executing date(1) to get the current time, use $SECONDS:

       SECONDS
              Each time this parameter is referenced, the
              number of seconds since shell invocation
              is returned.  If  a  value  is  assigned
              to  SECONDS,  the  value returned  upon
              subsequent references is the number of
              seconds since the assignment plus the
              value assigned.  If SECONDS is unset, it
              loses its special properties, even if it is
              subsequently reset.

Like so:

  throttle() {
    local -i limit
    ((limit = SECONDS + $1))
    while read line; do
      if ((limit < SECONDS)); then
        ((limit = SECONDS + $1))
        echo "$line"
      fi
    done
  }

Try it like so:

  $ yes | throttle 2

Of course, that's a bad example in that yes(1) will go as fast as you let it and it always outputs the same thing. In the case of yes(1) the better thing to do would be to sleep between reads, but yes(1) is a special case.