Applies To:
  • CitectSCADA 4.xx, 5.xx
Summary:
How does Windows and Citect handle daylight saving time and time zones and why does Citect sometimes display historical data such as Trends and Alarms with the wrong time?

Solution:
In 32bit versions of Windows ( eg. Windows 95 and Windows NT ) all time calculations and file time stamps are in UTC ( Universal Coordinate Time, which is exactly the same anywhere in the world and does not change during daylight savings ). Citect also keeps track of time in UTC.

When the user adjusts the time in Windows to the correct local time, the time is then converted by Windows back to UTC and this is the time that is used by Citect. If any of the time zone settings or daylight savings time corrections are incorrect, then the time Citect is using will actually be incorrect. It is therefore possible to have Citect PCs on a network that all display the same correct local time in Windows, but have different times for the display of historical information.

See also KB Q1977 for information on time calculation in the Citect Trend system.

In 16bit versions of Windows (eg. Windows for Workgroups) the time zone is adjusted by the TZ parameter in the Autoexec.bat file. In 32bit versions of Windows the settings are made via the Control Panel and saved in the Registry. 
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