How do you calculate the date with tick()? (Solved)

I figured it out, I'll have to have a go at working with leap years.

I know you can get the time like this:

    local a = tick()
    local hour = math.floor(a/60/60%24)
    local minute = math.floor(a/60%60)
    local second = math.floor(a%60)

But what's the math to calculate the date?