This works in 6.03 and above. Note this is your idle time as of the initialization of the dialog. It does not update automatically.
Code:
 dialog idle_time {
  title "My Idle Time"
  size -1 -1 60 20
  option dbu
  edit "", 1, 5 5 50 10, read
  button "Button", 3, 5 3 37 12, hide default ok cancel
}
menu * {
  &My Idle Time : dialog -m idle_time idle_time
}
on *:dialog:idle_time:init:*:{
  did -ra $dname 1 $duration($idle)
}