Example implementation of filesystem::file_clock which covers the range and precision of ext4 (https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout)

test_file_clock.cpp

#include "date/tz.h"
#include <ostream>
#include <istream>

namespace filesystem
{

struct file_clock
{
    using duration                  = std::chrono::nanoseconds;
    using rep                       = duration::rep;
    using period                    = duration::period;
    using time_point                = std::chrono::time_point<file_clock>;
    static constexpr bool is_steady = false;

    static time_point now();

    template<typename Duration>
    static
    std::chrono::time_point<std::chrono::system_clock, Duration>
    to_sys(const std::chrono::time_point<file_clock, Duration>& t) noexcept;

    template<typename Duration>
    static
    std::chrono::time_point<file_clock, Duration>
    from_sys(const std::chrono::time_point<std::chrono::system_clock, Duration>& t) noexcept;

    template<typename Duration>
    static
    std::chrono::time_point<date::local_t, Duration>
    to_local(const std::chrono::time_point<file_clock, Duration>& t) noexcept;

    template<typename Duration>
    static
    std::chrono::time_point<file_clock, Duration>
    from_local(const std::chrono::time_point<date::local_t, Duration>& t) noexcept;

    // private helpers

    static
    timespec
    to_timespec(const time_point& t) noexcept;

    static
    time_point
    from_timespec(const timespec& t) noexcept;
};

template <class Duration>
    using file_time = std::chrono::time_point<file_clock, Duration>;

using file_time_type = file_clock::time_point;

template <class Duration>
inline
std::chrono::time_point<std::chrono::system_clock, Duration>
file_clock::to_sys(const std::chrono::time_point<file_clock, Duration>& t) noexcept
{
    using namespace date;
    return sys_time<Duration>{t.time_since_epoch()} +
                             (sys_days{2174_y/1/1} - sys_days{1970_y/1/1});
}

template <class Duration>
inline
std::chrono::time_point<file_clock, Duration>
file_clock::from_sys(const std::chrono::time_point<std::chrono::system_clock, Duration>& t) noexcept
{
    using namespace date;
    return file_time<Duration>{t.time_since_epoch()} -
                              (sys_days{2174_y/1/1} - sys_days{1970_y/1/1});
}

template <class Duration>
inline
std::chrono::time_point<date::local_t, Duration>
file_clock::to_local(const std::chrono::time_point<file_clock, Duration>& t) noexcept
{
    using namespace date;
    return local_time<Duration>{to_sys(t).time_since_epoch()};
}

template <class Duration>
inline
std::chrono::time_point<file_clock, Duration>
file_clock::from_local(const std::chrono::time_point<date::local_t, Duration>& t) noexcept
{
    using namespace date;
    return file_time<Duration>{from_sys(sys_time<Duration>{t.time_since_epoch()})};
}

file_clock::time_point
file_clock::now()
{
    return from_sys(std::chrono::system_clock::now());
}

template <class CharT, class Traits, class Duration>
std::basic_ostream<CharT, Traits>&
to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
          const file_time<Duration>& t)
{
    using namespace std::chrono;
    const std::string abbrev("UTC");
    constepxr std::chrono::seconds offset{0};
    using D128 = duration<__int128, typename Duration::period>;
    return date::to_stream(os, fmt, file_clock::to_local(time_point_cast<D128>(t)),
                           &abbrev, &offset);
}

template <class Duration, class CharT, class Traits, class Alloc = std::allocator<CharT>>
std::basic_istream<CharT, Traits>&
from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
            file_time<Duration>& tp,
            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
            std::chrono::minutes* offset = nullptr)
{
    using namespace date;
    using namespace std::chrono;
    using D128 = duration<__int128, typename Duration::period>;
    local_time<D128> lp;
    from_stream(is, fmt, lp, abbrev, offset);
    if (!is.fail())
        tp = file_clock::from_local(lp);
    return is;
}

template <class CharT, class Traits, class Duration>
std::basic_ostream<CharT, Traits>&
operator<<(std::basic_ostream<CharT, Traits>& os, const file_time<Duration>& t)
{
    const CharT fmt[] = {'%', 'F', ' ', '%', 'T', CharT{}};
    return to_stream(os, fmt, t);
}

inline
timespec
file_clock::to_timespec(const time_point& t) noexcept
{
    using namespace date;
    using namespace std::chrono;
    auto tp = to_sys(time_point_cast<std::chrono::duration<__int128, std::nano>>(t));
    auto s = floor<seconds>(tp);
    timespec ts;
    ts.tv_sec = static_cast<decltype(ts.tv_sec)>(s.time_since_epoch().count());
    ts.tv_nsec = static_cast<decltype(ts.tv_nsec)>((tp - s).count());
    return ts;
}

inline
file_clock::time_point
file_clock::from_timespec(const timespec& t) noexcept
{
    using namespace date;
    using namespace std::chrono;
    auto d = std::chrono::duration<__int128>{t.tv_sec} + nanoseconds{t.tv_nsec};
    return time_point_cast<duration>(from_sys(sys_time<decltype(d)>{d}));
}

}  // namespace filesystem

#include <iostream>
#include <sstream>

int
main()
{
    using namespace std;
    std::cout << filesystem::file_clock::time_point::min() << '\n';
    std::cout << filesystem::file_clock::now() << '\n';
    std::cout << filesystem::file_clock::time_point::max() << '\n';
    std::istringstream in{"2466-04-11 23:47:16.854775807"};
    filesystem::file_clock::time_point tp;
    in >> date::parse("%F %T", tp);
    cout << tp << '\n';
    in.clear();
    in.str("1881-09-22 00:12:43.145224192");
    in >> date::parse("%F %T", tp);
    cout << tp << '\n';
    timespec ts = {15661036036, 854775807};  // or {-2785708037, 145224192}
    tp = filesystem::file_clock::from_timespec(ts);
    cout << tp << '\n';
    ts = filesystem::file_clock::to_timespec(tp);
    cout << "{" << ts.tv_sec << ", " << ts.tv_nsec << "}\n";
}

Example output:

1881-09-22 00:12:43.145224192
2018-08-01 21:32:51.875962000
2466-04-11 23:47:16.854775807
2466-04-11 23:47:16.854775807
1881-09-22 00:12:43.145224192
2466-04-11 23:47:16.854775807
{15661036036, 854775807}