Millisecond accurate time. I tried several concepts, most of it being wonky-calibration stuff that turned out to be inaccurate (micro leaps back in time). This is the one I am most satisfied with.

now.lua

local function clock()
    return (math.floor(os.clock() * 1000)/1000)
end

local _start_time = os.time()
local _start_clock = clock()
local _last
function now()
    
    -- Sanity check. See http://lua-users.org/wiki/SleepFunction
    -- >    -- warning: clock can eventually wrap around for sufficiently large n
    -- >    -- (whose value is platform dependent).  Even for n == 1, clock() - t0
    -- >    -- might become negative on the second that clock wraps.

    local _now_clock = clock();
    if _now_clock <= _start_clock then
      _start_time = os.time()
      _start_clock = _now_clock
    end
    
    local _return = _start_time + (clock() - _start_clock)
    
    -- Another sanity check, which could leave something like animations hanging. 
    -- If it does, you shouldn't be doing anything that is time sensitive on your
    -- target platform with lua.
    if _return < _last then
        return _last
    end
    
    _last = _return
    return _return
end
_last = now()

return now