i suggest to conenct signals in code not in the UI.. it gets hairy real fast when using UI.
just in the ready() function you connect them to your methods/functions.
func _ready():
var timer = get_node("Timer")
timer.timeout.connect(_on_timer_timeout)
in your case as it is build in signal you just use that built in signal name
func _ready():
button_down.connect(_on_timer_timeout)
func _on_timer_timeout():
pass