wMilliseconds

wMilliseconds is a field in the Windows API SYSTEMTIME structure used to store the milliseconds component of a time value. It is a 16-bit unsigned integer (WORD) that holds values from 0 to 999, representing the portion of the current second.

The SYSTEMTIME structure organizes date and time into eight fields: wYear, wMonth, wDayOfWeek, wDay, wHour, wMinute, wSecond, and wMilliseconds. wMilliseconds specifically captures the millisecond portion of the time, complementing the other fields to provide a human-readable date and time representation.

The field is populated by time retrieval functions such as GetSystemTime and GetLocalTime, which fill a SYSTEMTIME structure with the current UTC or local time, including the milliseconds value in wMilliseconds. It is also used in conversions between SYSTEMTIME and other time representations, such as FILETIME, via functions like SystemTimeToFileTime and FileTimeToSystemTime.

Usage considerations include time accuracy and normalization. System time resolution depends on the underlying hardware and operating system, so the milliseconds value reflects the clock’s granularity rather than guaranteeing precise sub-second timing. When performing arithmetic or formatting, systems typically ensure that wMilliseconds remains in the 0–999 range, with carry-overs handled in the broader time semantics.

Example: a program can call GetSystemTime(&st); and then read st.wMilliseconds to obtain the millisecond portion of the current time.

See also: SYSTEMTIME, GetSystemTime, GetLocalTime, SystemTimeToFileTime, FileTimeToSystemTime.