c++时间类

using XTimePoint = std::chrono::time_point<std::chrono::system_clock>;
using XTimeSpan  = std::chrono::seconds;

class XDataTime {
public:
    XDataTime() = default;
    XDataTime(const XTimePoint& time)
        : m_TimePoint(time)
    {
    }
    ~XDataTime() = default;

    XDataTime operator+(const XTimeSpan tt) const { return { m_TimePoint + tt }; }
    XTimeSpan operator-(const XDataTime dt) const { return { std::chrono::duration_cast<std::chrono::seconds>(m_TimePoint - dt.m_TimePoint) }; }

    tm        toTm() const
    {
        const time_t tt = std::chrono::system_clock::to_time_t(m_TimePoint);
        return *localtime(&tt);
    }

    char* toString(const char* format = nullptr) const
    {
        if(format == nullptr) {
            static char retStr[20];

            if(sprintf(retStr, "%d-%d%d-%d%d %d%d:%d%d:%d%d.%d", year(), month() / 10, month() % 10, day() / 10, day() % 10, hour() / 10, hour() % 10, minute() / 10, minute() % 10, second() / 10,
                       second() % 10, millisecond())
               < 0)
                return nullptr;
            return retStr;
        }
        return nullptr;
    }

    /// 当前时间.
    static XDataTime now() { return { std::chrono::system_clock::now() }; }
    /// 四位数年份
    int              year() const { return toTm().tm_year + 1900; }
    /// 两位数月份[1,12]
    int              month() const { return toTm().tm_mon + 1; }
    /// 两位数日期[1,28/29/30/31]
    int              day() const { return toTm().tm_mday; }
    /// 星期[1,7]
    int              weekday() const { return toTm().tm_wday; }
    /// 小时[0, 23]
    int              hour() const { return toTm().tm_hour; }
    /// 分钟[0, 59]
    int              minute() const { return toTm().tm_min; }
    /// 秒[0, 59]
    int              second() const { return toTm().tm_sec; }
    /// 毫秒[0,999]
    int              millisecond() const { return static_cast<int>(floor(nanoSecond() / pow(10, 6))); }
    /// 纳秒[0, 999999999]
    int              nanoSecond() const { return static_cast<int>(count() % std::nano::den); }
    /// 自1970-01-01 00:00:00起的纳秒数
    int64_t          count() const { return m_TimePoint.time_since_epoch().count(); }

private:
    XTimePoint m_TimePoint{};
};