Uptime of Erlang node

uptime.erl

-export([uptime/0, uptime/1, uptime_string/0]).

%% @doc uptime in native time units
uptime() ->
    erlang:monotonic_time() - erlang:system_info(start_time).

%% @doc uptime in specified time units
uptime(Unit) ->
    erlang:convert_time_unit(uptime(), native, Unit).

%% @doc uptime as binary formatted string
uptime_string() ->
    {D, {H, M, S}} = calendar:seconds_to_daystime(uptime(seconds)),
    list_to_binary(
      io_lib:format("~pd, ~p:~p:~p", [D, H, M, S])).

uptime2() ->
    {StartTime, _} = erlang:statistics(wall_clock),
    {D, {H, M, S}} = calendar:seconds_to_daystime(StartTime div 1000),
    list_to_binary(
      io_lib:format("~pd, ~p:~p:~p", [D, H, M, S])).