Hey! I know this is a super late response ( I just logged back into itch now for the GMTK Game Jam), and you probably already found an answer to your question elsewhere, but here's a little insight on how my timer works.
I have a script variable (gameTime) that tracks how many seconds are left in the round (starts at 30). I have a Timer node with a wait time of 1 second with One Shot set to false. I attached that node's "timeout" signal to my script, and every second that timer ticks and times out, I reduce the number of seconds left in the round by 1 (self.gameTime -= 1).
I use "self.gameTime" because I also have a setter configured for my gameTime variable. This makes it so every time that variable is updated, it also updates my Label node's text that shows how much time is left at the top of the screen.
Finally, the function that handles my Timer node's timeout also checks if gameTime <= 0, and if so, it runs some code that ends the round / game.
Hope that's helpful, and that you've been having fun with Godot!