SystemTimeToTzSpecificLocalTime

SystemTimeToTzSpecificLocalTime is a Windows API function that converts a UTC (Coordinated Universal Time) SystemTime structure into a local time based on a specified time‑zone context. It is declared in kernel32.h and implemented in Kernel32.dll. The function takes two parameters: a pointer to a TIME_ZONE_INFORMATION or *LOCAL* structure that describes the target time zone, and a pointer to the source SYSTEMTIME. It outputs the adjusted local time into a supplied SYSTEMTIME structure. The routine automatically accounts for daylight‑saving time rules, offset adjustments, and era changes, returning TRUE if the conversion succeeds or FALSE if the input is invalid or the time zone structure cannot be parsed.

The function is often used in applications that must display dates and times to users in their local zone or when serializing timestamps for transport across systems with differing zones. To use it, an application typically first obtains the relevant TIME_ZONE_INFORMATION by calling GetTimeZoneInformation, GetTimeZoneInformationForHandle, or by defining a custom time zone. It then populates a SYSTEMTIME object with UTC data, calls SystemTimeToTzSpecificLocalTime, and checks the return value. If the function fails, SetLastError provides a Windows error code for diagnostics.

Developers sometimes prefer using the newer function SystemTimeToTzSpecificLocalTimeEx, which accepts a TZSpecificLocalTime structure and allows for extended time‑zone definitions. Nevertheless, SystemTimeToTzSpecificLocalTime remains widely supported and documented, making it a reliable choice for legacy and cross‑platform Windows applications that need precise time‑zone handling.