How can I use RunService.Heartbeat to make a timer?

I'm not sure if Heartbeat is more effective or not than the while loop, but this is how I would do it:

local runService = game:GetService('RunService')

local maxTime = 1 -- when timer should end
local currentTime = maxTime

local function onFinished ()
	print('TIMER FINISHED')
end

runService.Heartbeat:Connect(function(dt)
	currentTime -= dt -- dt is delta time, which is the time between each frame

	if currentTime < 0 then 
		onFinished()
		currentTime = maxTime -- resets the timer
	end
end)

Heartbeat is only for the client, but for the server you would need something else.

Instead, we can use tick(). If we look up Tick() it returns the number of seconds (including milliseconds) from the Unix Epoch. So we can make a more accurate timer for the server using the following code.

local function startTimer (timeLength)
	local startValue = tick()

	while (startValue + timeLength) > tick() do
		-- do nothing
		wait(0.00001)
	end

	return 
end

The code above should be very accurate, and if you want it to be more accurate just decrease the wait() even more. But it would be very costly to do so. I am unsure of any better ways to make a timer, so correct me if I'm wrong.

EDIT: SORRY! My mistake, both would work on the server. Thanks to OfficerBrah for correcting me.