Code:
menu * {
  -
  Away: if (%brb == $null) /dialog -m brb brb | if (%brb != $null) /back
  -
}
dialog brb {
  title "Away"
  size -1 -1 220 85

  text "Message:", 1, 1 5 80 20
  edit "", 2, 80 3 148 20, autohs

  text "Expect Back:", 3, 1 25 80 20
  edit "", 4, 80 23 148 20, autohs

  text "Reason:", 5, 1 45 80 20
  edit "", 6, 80 43 148 20, autohs

  button "Away", 7, 1 65 109 20, ok
  button "Cancel", 8, 109 65 109 20, cancel

}
alias back {
  if (%back <= 2) || (%back == 4) {
    /amsg 12[10Back12] $chr(124) 12[10Time Gone12]: $duration($calc(($ctime - $ctime(%brb))))
    unset %reason
    unset %brb
    unset %back
  }
  elseif (%back == 3) {
    /amsg 12[10Back12] $chr(124) 12[10Time Gone12]: $duration($calc(($ctime - $ctime(%brb))))
    unset %brb
    unset %reason
    unset %back
  }
}
on *:dialog:brb:sclick:7: {
  if ($did(2) == $null) || ($did(4) != $null) || ($did(6) != $null) {
    /amsg 12[10Away12] $chr(124) 12[10Expected Back12] $did(4) $chr(124) 12[10Reason12] $did(6)
    set %reason $did(6)
    set %brb $fulldate
    set %back 1
  }
  elseif ($did(2) != $null) || ($did(4) == $null) || ($did(6) != $null) {
    /amsg Message: $did(2) $chr(124) Away. $chr(124) Reason: $did(6)
    set %brb $fulldate
    set %reason $did(6)
    set %back 2
  }
  elseif ($did(2) != $null) || ($did(4) != $null) || ($did(6) == $null) {
    /amsg Message: $did(2) $chr(124) Away. $chr(124) Expected Back: $did(4)
    set %brb $fulldate
    set %back 3
  }
  else {
    /amsg Message: $did(2) $chr(124) Away. $chr(124) Expected Back: $did(4) $chr(124) Reason: $did(6)
    set %reason $did(6)
    set %back 4
    set %brb $fulldate
  }
}