-- dayLength defines how long, in minutes, a day in your game is. Feel free to alter it.
local dayLength = 12

local daysPassed = 0

local cycleTime = dayLength*60
local minutesInADay = 24*60

local lighting = game:GetService("Lighting")

local startTime = tick() - (lighting:getMinutesAfterMidnight() / minutesInADay)*cycleTime
local endTime = startTime + cycleTime

local timeRatio = minutesInADay / cycleTime

if dayLength == 0 then
    dayLength = 1
end

repeat
    local currentTime = tick()

    if currentTime > endTime then
        startTime = endTime
        endTime = startTime + cycleTime
        --
        daysPassed = daysPassed + 1
        if daysPassed % 3 == 0 then --// every 3 days
            print 'its been 3 days'
        end
    end

    lighting:setMinutesAfterMidnight((currentTime - startTime)*timeRatio)
    wait(1/15)
until false