proc some_calc { timestamp size } {
  foreach {hour minute second} [split $timestamp ":"] {}
  set time [expr ( $hour * 3600 ) + ( $minute * 60 ) + $second]
  set size [expr $size * 1024]
  set bitrate [expr ($size / $time) * 8]
  puts "$bitrate kbit/s and [expr $bitrate / 8] kbyte/s"
}