System Time
System time is the current date and time of day. Windows keeps system time so that your
applications have ready access to accurate time. When Windows first starts, it
sets the system time to a value based on the real-time clock of the computer and
thereafter regularly updates the time.
Windows bases system time on
coordinated universal time (UTC). UTC-based time is loosely defined as the current date and time of day
in Greenwich, England. You can retrieve the system time by using the
GetSystemTime function.
GetSystemTime copies the time to a
SYSTEMTIME structure that contains individual members for month, day, year, weekday,
hour, minute, second, and milliseconds.
Because system time is UTC-based, the time copied to the
SYSTEMTIME structure may not be the same as the
local time 
the date and time of day for your time zone. You can retrieve the local time
by using the
GetLocalTime function.
GetLocalTime converts the system time to a local time based on the current time-zone
settings and copies the result to a
SYSTEMTIME structure. To convert a UTC time stored in a
SYSTEMTIME structure to a local time, use the
SystemTimeToTzSpecificLocalTime function.
The current time-zone settings control how Windows converts between UTC and
local time. You can retrieve the current time-zone settings by using the
GetTimeZoneInformation function. The function copies the setting to a
TIME_ZONE_INFORMATION structure and returns a value indicating whether local time is currently in
standard or daylight-savings time. You can set the time-zone settings by using
the
SetTimeZoneInformation function.
You can set the system time by using the
SetSystemTime or
SetLocalTime function.
SetSystemTime assumes you have specified a UTC-based time;
SetLocalTime assumes local time and converts to UTC before setting the system time. If you
change the system time, you should send the
WM_TIMECHANGE message to all top-level windows by using the
SendMessage function.
The
GetSystemTimeAdjustment and
SetSystemTimeAdjustment functions support algorithms that synchronize the time-of-day clock with
another time source using a periodic time adjustment applied at each clock
interrupt.
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