How do I calculate the time in which the process completes the request

Question

First of all thank you for looking at my question. Create a server that manages a list of available frequencies(say PROCESS [1,2,3,4,5,6,7,8,9,10]. The client process should be able to request any available frequency and release it. When the client releases the frequency, the server would calculate the amount (5 Rs/Sec) based upon the time the frequency was used by the client and would send the bill to the client.

I really do appreciate your help even if you can’t solve. I have been trying this for 1 whole month and I’m honestly so down and hopeless. Thank you so much.

-module(b).

-compile([export_all, nowarn_export_all]).

run() ->
    file:delete("example.dets"),
    case dets:info(example, owner) of
        undefined ->
            dets:open_file(example, [{file, "example.dets"}]);
        _ ->
            skip
    end,
    spawn(fun() -> init() end),
    timer:sleep(100),
    Client = self(),

    example_server ! {allocate, Client, aaa, 5},
    example_server ! {allocate, Client, bbb, 5},
    example_server ! {allocate, Client, aaa, 5},
    timer:sleep(1000),
    example_server ! {release, Client, aaa, 5},
    example_server ! {release, Client, aaa, 5},
    timer:sleep(100),
    io:format("~p~n", [receive_server_msg([])]),

    example_server ! {allocate, Client, aaa, 5},
    timer:sleep(100),
    exit(whereis(example_server), kill),
    spawn(fun() -> init() end),
    timer:sleep(100),
    example_server ! {allocate, Client, bbb, 5},
    example_server ! {allocate, Client, aaa, 5},
    timer:sleep(1000),
    example_server ! {release, Client, aaa, 5},
    example_server ! {release, Client, aaa, 5},
    timer:sleep(100),
    io:format("~p~n", [receive_server_msg([])]),
    example_server ! {stop},

    dets:close(example).

receive_server_msg(List) ->
    receive
        Reply ->
            receive_server_msg([Reply|List])
        after 0 ->
            lists:reverse(List)
    end.
            

init() ->
    erlang:register(example_server, self()),
    State = other_data_you_need,
    manager(State).

manager(State) ->
    receive
        {stop} -> ok;
        {allocate, Requestor, Name, Frequency} ->
            case check_allocate_request_and_modify_state(Name, Frequency, State) of
                {ok, State1} ->
                    Requestor ! {grant, Frequency},
                    manager(State1);
                {error, Reason} ->
                    Requestor ! {error, Reason},
                    manager(State)
            end;
        {release, Requestor, Name, Frequency} ->
            case check_release_request_and_modify_state(Name, Frequency, State) of
                {ok, Cost, State1} ->
                    Requestor ! {bill, Cost},
                    manager(State1);
                {error, Reason} ->
                    Requestor ! {error, Reason},
                    manager(State)
            end;
        UnknownMsg ->
            logger:error("unknown message: ~p", [UnknownMsg]),
            manager(State)
    end.

check_allocate_request_and_modify_state(Name, Frequency, State) ->
    case lists:member(Frequency, [1,2,3,4,5,6,7,8,9,10]) of
        true ->
            case dets:lookup(example, Frequency) of
                [] ->
                    dets:insert(example, {Frequency, Name, erlang:system_time(second)}),
                    {ok, State};
                [{_, Name, _}] ->
                    {error, using};
                _ ->
                    {error, other_using}
            end;
        _ ->
            {error, bad_request}
    end.

check_release_request_and_modify_state(Name, Frequency, State) ->
    case dets:lookup(example, Frequency) of
        [{_, Name, StartTime}] ->
            dets:delete(example, Frequency),
            Cost = (erlang:system_time(second) - StartTime),% * Price,
            {ok, Cost, State};
        _ ->
            {error, not_using}
    end.

If possible could you help me correct this and help me with the right format or right answer.