标题: VARIANT与VT_DATE
作者: Demon
链接: https://demon.tw/programming/variant-vt_date.html
版权: 本博客的所有文章，都遵守“署名-非商业性使用-相同方式共享 2.5 中国大陆”协议条款。
VARIANT结构的定义在OAIDL.H中可以找到：
typedef /* [wire_marshal] */ struct tagVARIANT VARIANT;
struct tagVARIANT
{
union
{
struct __tagVARIANT
{
VARTYPE vt;
WORD wReserved1;
WORD wReserved2;
WORD wReserved3;
union
{
LONGLONG llVal;
LONG lVal;
BYTE bVal;
SHORT iVal;
FLOAT fltVal;
DOUBLE dblVal;
VARIANT_BOOL boolVal;
_VARIANT_BOOL bool;
SCODE scode;
CY cyVal;
DATE date;
BSTR bstrVal;
IUnknown *punkVal;
IDispatch *pdispVal;
SAFEARRAY *parray;
BYTE *pbVal;
SHORT *piVal;
LONG *plVal;
LONGLONG *pllVal;
FLOAT *pfltVal;
DOUBLE *pdblVal;
VARIANT_BOOL *pboolVal;
_VARIANT_BOOL *pbool;
SCODE *pscode;
CY *pcyVal;
DATE *pdate;
BSTR *pbstrVal;
IUnknown **ppunkVal;
IDispatch **ppdispVal;
SAFEARRAY **pparray;
VARIANT *pvarVal;
PVOID byref;
CHAR cVal;
USHORT uiVal;
ULONG ulVal;
ULONGLONG ullVal;
INT intVal;
UINT uintVal;
DECIMAL *pdecVal;
CHAR *pcVal;
USHORT *puiVal;
ULONG *pulVal;
ULONGLONG *pullVal;
INT *pintVal;
UINT *puintVal;
struct __tagBRECORD
{
PVOID pvRecord;
IRecordInfo *pRecInfo;
} __VARIANT_NAME_4;
} __VARIANT_NAME_3;
} __VARIANT_NAME_2;
DECIMAL decVal;
} __VARIANT_NAME_1;
} ;
typedef VARIANT *LPVARIANT;
typedef VARIANT VARIANTARG;
typedef VARIANT *LPVARIANTARG;
当VARIANT的类型vt为VT_DATE时，有效的域为date，其数据类型为DATE，DATE的定义在WTYPES.H中：
typedef double DATE;
可以看出，VT_DATE的实际类型是double，那么如何用double来表示时间的呢？MSDN上是这么说的：
A date and time value. Dates are represented as double-precision numbers, where midnight, January 1, 1900 is 2.0, January 2, 1900 is 3.0, and so on.
没人看得懂这是什么意思，还有一种说法是：
The OLE automation date format is a floating point value, counting days since midnight 30 December 1899. Hours and minutes are represented as fractional days.
遗憾的是仍然没人看得懂，人类能看懂的说法是：
An OLEAUT date is a double where the signed integer part is the number of days since 30 December 1899 and the fraction part is the amount of that day gone by. So what is 1.75? That’s 6PM , 31 Dec 1899. What about -1.75? That’s 6 PM 29 Dec 1899.
参考链接：
- http://msdn.microsoft.com/zh-cn/library/windows/desktop/ms221627%28v=vs.85%29.aspx
- http://blogs.msdn.com/b/ericlippert/archive/2003/09/16/eric-s-complete-guide-to-vt-date.aspx
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