I see, time_left is a inbuilt function used to call the Timer.
I played around with the Timer and this is probably easiest for your purpose. Just convert it to an integer and then back to a string.
func _process(delta):
var time = str($Timer.time_left)
var roundtime = int(time)
$Label.text = str(roundtime)
You can use the round function too, with a slightly more complicated method if you really need to. But this should suffice.