Issue when scheduling tasks using clock() function

If I was to implement some timer mechanism in C++, I would probably be using the std::chrono namespace together with std::priority_queue.

#include <functional>
#include <queue>
#include <chrono>
#include <sys/time.h>  // for `time_t` and `struct timeval`

namespace events
{
    struct event
    {
        typedef std::function<void()> callback_type;
        typedef std::chrono::time_point<std::chrono::system_clock> time_type;

        event(const callback_type &cb, const time_type &when)
            : callback_(cb), when_(when)
            { }

        void operator()() const
            { callback_(); }

        callback_type callback_;
        time_type     when_;
    };

    struct event_less : public std::less<event>
    {
        bool operator()(const event &e1, const event &e2) const
        {
            return (e2.when_ < e1.when_);
        }
    };

    std::priority_queue<event, std::vector<event>, event_less> event_queue;

    void add(const event::callback_type &cb, const time_t &when)
    {
        auto real_when = std::chrono::system_clock::from_time_t(when);

        event_queue.emplace(cb, real_when);
    }

    void add(const event::callback_type &cb, const timeval &when)
    {
        auto real_when = std::chrono::system_clock::from_time_t(when.tv_sec) +
                         std::chrono::microseconds(when.tv_usec);

        event_queue.emplace(cb, real_when);
    }

    void add(const event::callback_type &cb,
             const std::chrono::time_point<std::chrono::system_clock> &when)
    {
        event_queue.emplace(cb, when);
    }

    void timer()
    {
        event::time_type now = std::chrono::system_clock::now();

        while (!event_queue.empty() &&
               (event_queue.top().when_ < now))
        {
            event_queue.top()();
            event_queue.pop();
        }
    }
}

To use, simply add events using events::add, and call events::timer a few times every second.

Simple example:

void foo()
{
    std::cout << "hello from foo\n";
}

void done()
{
    std::cout << "Done!\n";
}

struct bar
{
    void hello()
    {
        std::cout << "Hello from bar::hello\n";
    }
};

auto now = std::chrono::system_clock::now();
bar b;

events::add(foo, now + std::chrono::seconds(2));

events::add(std::bind(&bar::hello, b), now + std::chrono::seconds(4));

events::add(done, now + std::chrono::seconds(6));

while (true)
{
    usleep(10000);  // TODO: Do some "real" work here instead...
    events::timer();
}

The above example will print:

hello from foo
hello from bar::hello
Done!

One line will be printed every two second. After "Done!" the program will just loop forever, doing nothing.

Note that this program contains lots of C++11 functionality, but has been tested with GCC 4.4.5 and 4.7.1. VC++2010 unfortunately does not have the <chrono> header, but the VC++2012RC apparently have it.