Time.deltaTime for Roblox Lua?

While you can only calculate the per-renderframe delta time on the Client (because only LocalScripts can use RenderStepped), you can implement delta time using tick():

local last = tick()
while true do
    local t = tick()
    local dt = t - last
    last = t

    print(tick)
    print(dt)

    wait() --waits 1/30th of a second, usually.
    --game:GetService("RunService").RenderStepped:wait() --waits the exact amount of time it takes to create the next frame. Maxes at 1/60th of a second.
end

EDIT: For future explorers:

game:GetService("RunService").Heartbeat:wait() updates with the physics handling on the Server, which is slightly faster than what wait() on its own will give you.