The SupportsDST property retrieves a value that indicates whether a Time Zone supports Daylight-Saving Time.
All other ITimeZone properties are defined only if the SupportsDST property is VARIANT_TRUE.
The VARIANT_BOOL data type (also known as VT_BOOL) uses the value -1 to represent TRUE and 0 to represent FALSE. It is defined in the wtypes.h header file in the following manner:
typedef short VARIANT_BOOL;
A common programming error occurs when, for readability purposes, you want to set a VARIANT_BOOL value to TRUE or FALSE. You can achieve the same effect by using the aliases VARIANT_TRUE and VARIANT_FALSE, which are also defined in the wtypes.h header file.
#define VARIANT_TRUE ((VARIANT_BOOL)0xffff) #define VARIANT_FALSE ((VARIANT_BOOL)0)