This is WindowsSystemTime structure (equivalent to the unmanaged SYSTEMTIME) structure, but stores NOT a valid date. Instead, the following rules apply: If "month" is 0, there is no daylight savings time for this zone. The dayOfWeek value indicates the day of the week on which daylight savings time reverts back to standard time. The day value indicates, starting at 1 (with a maximum value of 5, saying "last") the occurrence of that weekday within the month on which the daylight savings time reverts back.