VariantTimeToSystemTime 实现源码

VariantTimeToSystemTime 函数是将 Variant 时间值转换为系统时间值的函数。以下是该函数的实现源码：

BOOL VariantTimeToSystemTime(DATE dt, LPSYSTEMTIME lpSystemTime)
{
    if (lpSystemTime == NULL)
        return FALSE;

    // 日期的整数部分是自 1899 年 12 月 30 日以来的天数
    double dTemp = dt + HALF_SECOND / SECONDS_PER_DAY;

    // 这个数字表示了 1900 年 1 月 1 日的日期
    // 也就是说，它是 dTemp 中减去 2 的结果
    int nDays = (int)dTemp;

    // 将时间部分转换为 SYSTEMTIME 结构
    double dblSecsInDay = dTemp - (double)nDays;
    int nSeconds = (int)(dblSecsInDay * SECONDS_PER_DAY);

    lpSystemTime->wHour = nSeconds / SECONDS_PER_HOUR;
    nSeconds -= lpSystemTime->wHour * SECONDS_PER_HOUR;
    lpSystemTime->wMinute = nSeconds / SECONDS_PER_MINUTE;
    nSeconds -= lpSystemTime->wMinute * SECONDS_PER_MINUTE;
    lpSystemTime->wSecond = nSeconds;
    lpSystemTime->wMilliseconds = (WORD)((dblSecsInDay - (double)nSeconds) * 1000.0);

    // 将日期部分转换为 SYSTEMTIME 结构
    // 闰年必须是 4 的倍数，但世纪闰年必须是 400 的倍数
    int nLeapYears = (nDays - DAYS_FROM_BASELINE_TO_1900) / 1461;
    int nYear = 1900 + nLeapYears * 4;
    int nDaysInLeapYear = (nDays - DAYS_FROM_BASELINE_TO_1900) % 1461;
    for (;;)
    {
        int nDaysInYear = DaysInYear(nYear);
        if (nDaysInLeapYear < nDaysInYear)
            break;
        nYear++;
        nDaysInLeapYear -= nDaysInYear;
    }

    lpSystemTime->wYear = (WORD)nYear;
    lpSystemTime->wMonth = 1;

    int nMonth;
    for (nMonth = 1; nMonth <= 12; nMonth++)
    {
        int nDaysInMonth = DaysInMonth(nMonth, IsLeapYear(nYear));
        if (nDaysInLeapYear < nDaysInMonth)
            break;
        nDaysInLeapYear -= nDaysInMonth;
        lpSystemTime->wMonth++;
    }

    lpSystemTime->wDay = (WORD)(nDaysInLeapYear + 1);
    lpSystemTime->wDayOfWeek = (WORD)((nDays + 1) % 7);

    return TRUE;
}

这段代码中包含了一些常量和辅助函数，它们的定义如下：

#define DAYS_FROM_BASELINE_TO_1900  693593
#define SECONDS_PER_DAY             86400
#define SECONDS_PER_HOUR            3600
#define SECONDS_PER_MINUTE          60
#define HALF_SECOND                 (1.0 / (2.0 * SECONDS_PER_DAY))

inline BOOL IsLeapYear(int nYear)
{
    return ((nYear & 3) == 0) && ((nYear % 100) != 0 || (nYear % 400) == 0);
}

inline int DaysInMonth(int nMonth, BOOL bLeapYear)
{
    static int nDaysInMonth[] =
    {
        31, 28, 31, 30, 31, 30,
        31, 31, 30, 31, 30, 31
    };
    return (nMonth == 2 && bLeapYear) ? 29 : nDaysInMonth[nMonth - 1];
}

inline int DaysInYear(int nYear)
{
    return IsLeapYear(nYear) ? 366 : 365;
}