The StandardDay property specifies the day of the StandardMonth when the transition from daylight savings time to standard ...

The StandardDay property specifies the day of the StandardMonth when the transition from daylight savings time to standard time occurs on this operating system.

Example: 31 
The property specifies a SYSTEMTIME structure that contains a date and local time when the transition from daylight time to standard time occurs on this operating system. If this date is not specified, the wMonth member in the SYSTEMTIME structure must be zero. If this date is specified, the DaylightDate value in the TIME_ZONE_INFORMATION structure must also be specified. 
This member supports two date formats. Absolute format specifies an exact date and time when standard time begins. In this form, the wYear, wMonth, wDay, wHour, wMinute, wSecond, and wMilliseconds members of the SYSTEMTIME structure are used to specify an exact date. 
Day-in-month format is specified by setting the wYear member to zero, setting the wDayOfWeek member to an appropriate weekday, and using a wDay value in the range 1 through 5 to select the correct day in the month. Using this notation, the first Sunday in April can be specified, as can the last Thursday in October (5 is equal to \"the last\").