如何在DATE与SYSTEMTIME之间转化
typedef double DATE;
DATE变量可以直接赋值给_variant_t对象，在用ADO访问数据库的时候，需要在_variant_t和SYSTEMTIME之间转换，MS提供了如下两个函数：
SystemTimeToVariantTime
SystemTimeToVariantTime
Converts a system time to a variant representation.
INT SystemTimeToVariantTime( LPSYSTEMTIME lpSystemTime, double *pvtime );
Parameters
- lpSystemTime
- The system time.
- pvtime
- Returned variant time.
Return Value
The function returns one of the following:
| Value | Meaning |
|---|---|
| zero | Failure. |
| non-zero | Success. |
Comments
A variant time is stored as an 8-byte real value (double), representing a date between January 1, 100 and December 31, 9999, inclusive. The value 2.0 represents January 1, 1900; 3.0 represents January 2, 1900, and so on. Adding 1 to the value increments the date by a day. The fractional part of the value represents the time of day. Therefore, 2.5 represents noon on January 1, 1900; 3.25 represents 6:00 A.M. on January 2, 1900, and so on. Negative numbers represent dates prior to December 30, 1899.
The variant time resolves to one second. Any milliseconds in the input date are ignored.
The LPSYSTEMTIME structure is useful for the following reasons:
- It spans all time/date periods. MS-DOS date/time is limited to representing only those dates between 1/1/1980 and 12/31/2107.
- The date/time elements are all easily accessible without needing to do any bit decoding.
- The National Data Support data and time formatting functions GetDateFormat and GetTimeFormat take a LPSYSTEMTIME value as input. For more information, see the COM Programmer’s Reference in the Platform SDK.
- It is the default Win32 time/date data format supported by Windows NT and Windows 95.
The SystemTimeToVariantTime function will accept invalid dates and try to fix them when resolving to a VARIANT time. For example, an invalid date such as 2/29/2001 will resolve to 3/1/2001. Only days are fixed, so invalid month values result in an error being returned. Days are checked to be between 1 and 31. Negative days and days greater than 31 results in an error. A day less than 31 but greater than the maximum day in that month has the day promoted to the appropriate day of the next month. A day equal to zero resolves as the last day of the previous month. For example, an invalid dates such as 2/0/2001 will resolve to 1/31/2001.
Requirements
Windows NT/2000: Requires Windows NT 3.1 or later.
Windows 95/98: Requires Windows 95 OSR2 or later.
Header: Declared in oleauto.h.
Library: Use oleaut32.lib.
VariantTimeToSystemTime
VariantTimeToSystemTime
Converts the variant representation of time to system time values.
INT VariantTimeToSystemTime( double vtime, LPSYSTEMTIME lpSystemTime );
Parameters
- vtime
- The variant time that will be converted.
- lpSystemTime
- Pointer to the location where the converted time will be stored.
Return Value
The return value obtained from the returned INT is one of the following.
| Value | Meaning |
|---|---|
| TRUE | Success. |
| FALSE | Failure. |
Comments
A variant time is stored as an 8-byte real value (double), representing a date between January 1, 100 and December 31, 9999, inclusive. The value 2.0 represents January 1, 1900; 3.0 represents January 2, 1900, and so on. Adding 1 to the value increments the date by a day. The fractional part of the value represents the time of day. Therefore, 2.5 represents noon on January 1, 1900; 3.25 represents 6:00 A.M. on January 2, 1900, and so on. Negative numbers represent the dates prior to December 30, 1899.
Using the SYSTEMTIME structure is useful because:
- It spans all time/date periods. MS-DOS date/time is limited to representing only those dates between 1/1/1980 and 12/31/2107.
- The date/time elements are all easily accessible without needing to do any bit decoding.
- The National Language Support data and time formatting functions GetDateFormat and GetTimeFormat take a SYSTEMTIME value as input.
- It is the default Win32 time and date data format supported by Windows NT and Windows 95.
The VariantTimeToSystemTime function will accept invalid dates and try to fix them when resolving to a VARIANT time. For example, an invalid date such as 2/29/2001 will resolve to 3/1/2001. Only days are fixed, so invalid month values result in an error being returned. Days are checked to be between 1 and 31. Negative days and days greater than 31 results in an error. A day less than 31 but greater than the maximum day in that month has the day promoted to the appropriate day of the next month. A day equal to zero resolves as the last day of the previous month. For example, an invalid dates such as 2/0/2001 will resolve to 1/31/2001.
Requirements
Windows NT/2000: Requires Windows NT 3.1 or later.
Windows 95/98: Requires Windows 95 OSR2 or later.
Header: Declared in oleauto.h.
Library: Use oleaut32.lib.