Day/Night Cycle System can't go any slower?

Right now I would like the day cycle to be slower, but it seems that I can't make it much slower.

while RS.Stepped:Wait() do
	local Clock = Lighting.ClockTime
	local Speed
	if (Clock > 5 and Clock < 8) or (Clock > 16 and Clock < 20) then
		Speed = 20 -- Transitions go the fastest
	end
	if Clock >= 20 and Clock <= 6 then
		Speed = 1 -- Nights are shorter than days
	else
		Speed = 0.0003
	end
	Lighting.ClockTime = Clock + Speed
end

0.0003 is the current speed set for Day, which means the clock time increases by roughly 1 every minute. Reducing this 0.0003 value will break the system since I assume the values are too low.

But how would I make the day time slower if reducing this 0.0003 value won't work?

You don't need to add to the speed on every step. You can use a modulus to reduce the speed even further.

local StepCount = 0

while RS.Stepped:Wait() do
    StepCount = StepCount + 1
    local Clock = Lighting.ClockTime
    local Speed
    if (Clock > 5 and Clock < 8) or (Clock > 16 and Clock < 20) then
        Speed = 20 -- Transitions go the fastest
    end
    if Clock >= 20 and Clock <= 6 then
        Speed = 1 -- Nights are shorter than days
    else
        if StepCount % 10 == 0 then
            Speed = 0.0003
        else
            Speed = 0
        end
    end
    Lighting.ClockTime = Clock + Speed
    if StepCount % 10 == 0 then
            StepCount = 0 --To prevent floating errors with extremely large step counts
    end
end

In this case only every 10th step will the clock change. We need to remember what can happen with arithmetic in computers, so to adjust for potential integer overflows we have reset the step count as well every 10th step. This needs to be done with a separate if statement because we can't guarantee that the StepCount wouldn't overflow in the other sections of code without doing further math, so it does add an extra if statement.