How do I keep track a countdown time and display it?

Fire a local script with remote events whenever the timer starts. In one of the parameters you can put how long the cooldown is going to be, so the local script knows where to start it. Make another parameter to let the local script know which text to change. Oh and yeah, another one showing what the original text was.

Here's what the local script would look like:

local repStorage = game:GetService("ReplicatedStorage")
local remoteEvent = repStorage:WaitForChild("RemoteEvent")

remoteEvent.OnClientEvent:Connect(function(cooldown,textToChange,originalText)

	local text = script.Parent:WaitForChild(textToChange) -- Make sure to send the name of textToChange instead of the actual text itself, or else this probably wouldn't work.

	local counter = cooldown
	
	while true do
		
		if counter <= 0 then

			text.Text = tostring(counter) -- This sets the text of the key to press to the counter.
			counter = counter - 0.1
			wait(0.1)

		else		
			
			text.Text = originalText -- This sets the text back to normal after it reaches 0.

		end

	end

end)

Server Script:

local repStorage = game:GetService("ReplicatedStorage")
local remoteEvent = repStorage:WaitForChild("RemoteEvent")

remoteEvent:FireClient(player,cooldown,text.Name,text.Text) -- Note that you're going to have to have the player as the first parameter when firing from a server script so that it knows which local player's local script to fire.

If you don't understand remote events, look at this video: https://www.youtube.com/watch?v=4Dc_bri9mjs