Tuesday, November 18, 2014

Standard Time, Daylight Saving Time and Local Time

The earth is round like a ball. When it's night in the US, it's morning in China. To do business beyond a local region, people need a common reference to communicate time. Standard time is the synchronization of clocks in different geographical locations within a time zone to a common time standard, usually based on the meridian at the center of the time zone.

Daylight saving time (DST) typically involves advancing clocks by an hour near the start of spring and adjusting back in fall. The primary purpose of daylight saving time is to adjust human activity schedule during day time to take advantage of the daylight hours during summer time. Not every country has adopted DST. In U.S., Arizona and Hawaii are not observing DST. In the regions where DST is adopted, the term standard time typically refers to the time without the offset for daylight saving time.

Local time means the time on the clock. In the regions not observing DST, local time is the same as standard time. In the regions observing DST, local time is standard time in winter and fall, daylight saving time in spring and summer. During the day when DST starts, there are 23 hours in local time. During the day when DST ends, there are 25 hours in local time.

In load forecasting, the load and weather data often comes from different sources. The misunderstanding of these terms often leads to data processing errors in load forecasting. Typical formats I've seen in load forecasting projects include 1) Greenwich Mean Time (GMT); 2) standard time; and 3) local time. While both GMT and standard time are observing 24-hour days, there are several variations of local time in load forecasting practice. During the first day of DST, some data sources observe a 23-hour day, while some use a 24-hour day with the 2nd (or 3rd) hour as zero or missing. During the last day of DST, some data sources observe a 25-hour day, while some observe a 24-hour day with the 2nd (or 3rd) hour as the sum of the two original readings of the same hour.

Regardless what format the raw data has, I usually convert the load and weather data to a 24-hour local time. At the beginning of the DST, I fill in the "missing" hour as the average between its adjacent two hours. At the end of DST, I average the original two readings in the same hour. For the convenience of the readers of my blog, below is a list of DST start and end days in the U.S. from 1987 to 2020.

Year

DST Start

DST End

1987

4/5/1987

10/25/1987

1988

4/3/1988

10/30/1988

1989

4/2/1989

10/29/1989

1990

4/1/1990

10/28/1990

1991

4/7/1991

10/27/1991

1992

4/5/1992

10/25/1992

1993

4/4/1993

10/31/1993

1994

4/3/1994

10/30/1994

1995

4/2/1995

10/29/1995

1996

4/7/1996

10/27/1996

1997

4/6/1997

10/26/1997

1998

4/5/1998

10/25/1998

1999

4/4/1999

10/31/1999

2000

4/2/2000

10/29/2000

2001

4/1/2001

10/28/2001

2002

4/7/2002

10/27/2002

2003

4/6/2003

10/26/2003

2004

4/4/2004

10/31/2004

2005

4/3/2005

10/30/2005

2006

4/2/2006

10/29/2006

2007

3/11/2007

11/4/2007

2008

3/9/2008

11/2/2008

2009

3/8/2009

11/1/2009

2010

3/14/2010

11/7/2010

2011

3/13/2011

11/6/2011

2012

3/11/2012

11/4/2012

2013

3/10/2013

11/3/2013

2014

3/9/2014

11/2/2014

2015

3/8/2015

11/1/2015

2016

3/13/2016

11/6/2016

2017

3/12/2017

11/5/2017

2018

3/11/2018

11/4/2018

2019

3/10/2019

11/3/2019

2020

3/8/2020

11/1/2020

 

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