How do I make a NumberValue into Hours,Minutes and Seconds?

I'm wanting to make a GUI on the side of your screen that will count down from 23:59:59 to 0 and will adjust depending on hours minutes and seconds, Here's the code for removing 1 from the value after 1 second:

script.Parent.Text = "".. script.Parent.Time.Value

while wait(1) do
    script.Parent.Time.Value = script.Parent.Time.Value - 1
end

This will probably get taken down but I just want to know how to do this because I have no idea how to demonstrate this any other way.