This past Sunday morning Daylight Saving Time ended at 2:00AM and the clocks rolled back an hour, giving us all some extra rest. However, for data process historians and engineers like yourself, this semi-annual phenomenon can be anything but relaxing! In fact as you have learned, the Daylight Saving Time (DST) scenario is one of the most difficult situations for a data historian to handle. Let me explain for others, not as familiar, while I answer your question.
Imagine you were to search for data at 1:35AM on the Sunday of DST. How would the computer know if you meant the first 1:35AM or the second 1:35AM of the morning? The Canary Historian solves this problem by storing all the data with UTC time stamps. UTC stands for Coordinated Universal Time and is the basis for civil time today. Note, this is a time standard, not a time zone.
Until 1972, Greenwich Mean Time (GMT), or Zulu Time, was the same as Universal Time. Since then, GMT is no longer a time standard but instead a time zone used by a few countries in Africa and Western Europe, including the UK during winter and Iceland year round. Neither UTC nor GMT ever change for Daylight Saving Time. However, some of the countries that use GMT switch to different time zones during their DST period. For example, the United Kingdom is not on GMT all year, it uses British Summer Time, which is one hour ahead of GMT, during the summer months.
Since all Canary data values are time stamped using UTC time, we do not compensate for DST transitions. Instead, it is the responsibility of client applications, like Axiom or the Excel Add-in, that are reading the data from the historian to properly interpret the UTC time stamp. The application simply applies the correct time zone as well as the appropriate DST rules depending on time of year to properly display the time stamp in the correct local time. Seems like a simple solution, right? Canary has successfully used this strategy since 1993 and it works great!
Many other historians still use a local time stamp when storing data values. I recently saw one vendor recommend their users shut the historian off and not collect data during the the DST transition window. Not a very nice solution to such a simple problem.
I smile knowing our clients never have a firestorm of issues related to DST transitions; I hope that helps them sleep as peacefully as I do.
Sincerely,
Gary Stern
President and Founder
Canary Labs