Daylight-Savings Time & Time-Zone Offset

Although legislation may soon make Daylight Savings Time “go away” in the USA (and become the new Standard Time), it isn't going away everywhere.

The NAESB REQ.21 ESPI (“Green Button”) standard includes <LocalTimeParameters>(from ESPI) (of type <TimeConfiguration>(from ESPI) ) to allow data to be shared using timestamps in Coordinated Universal Time (UTC) and denoting the offset of time through a time zone and a Daylight Savings Time (DST) offset.

Bitmap of dstStartRule and dstEndRule

This is the bitmap-encoded rule from which the start or end time—within the current year, to which daylight savings time offset must be applied—is calculated:

The 32-bit rule encoding:

• Bits 28 - 31 (4 bits):  month … 1 — 12
• Bits 25 - 27 (3 bits):  operator … detailed below
• Bits 20 - 24 (5 bits):  day of the month … 1 — 31 (where 0 = not applicable)
• Bits 17 - 19 (3 bits):  day of the week … 1 — 7 (where 1 = “Monday” and 0 = not applicable)
• Bits 12 - 16 (5 bits):  hours … 0 — 23
• Bits   0 - 11 (12 bits):  seconds … 0 — 3599

The operators:

• 0: DST starts/ends on the Day of the Month (a particular number date)
• 1: DST starts/ends on the Day of the Week that is on or after the Day of the Month
• 2: DST starts/ends on the first occurrence of the Day of the Week in a month
• 3: DST starts/ends on the second occurrence of the Day of the Week in a month
• 4: DST starts/ends on the third occurrence of the Day of the Week in a month
• 5: DST starts/ends on the forth occurrence of the Day of the Week in a month
• 6: DST starts/ends on the fifth occurrence of the Day of the Week in a month
• 7: DST starts/ends on the last occurrence of the Day of the Week in a month

DST Examples

DST Start Example:

Example of a North American DST-Start Rule:

In a particular area, DST starts on the second Sunday of March at 2:00 AM.  The rule is… 

Month: 3 (“March”)
Operator: 3 (“the second occurrence of the Day of the Week in a month”)
Day of Month: 0 (“not applicable”)
Day of Week: 7 (“Sunday”)
Hours: 2 (“2:__”)
Seconds: 0 ( “__:00” or 00 minutes)

Then, the hexadecimal is used in the dstEndRule:

<dstEndRule>ESPI360E2000</dstEndRule>ESPI

… to be placed into a <LocalTimeParameters>(from ESPI) element.

DST End Example:

For a DST-End Rule, here's example:

DST ends on the first Sunday of November at 2:00 AM.  The rule is… 

Month: 11 (“November”)
Operator: 2 (“the first occurrence of the Day of the Week in a month”)
Day of Month: 0 (“not applicable”)
Day of Week: 7 (“Sunday”)
Hours: 2 (“2:__”)
Seconds: 0 ( “__:00” or 00 minutes)

Then, the hexadecimal is used in the dstEndRule:

<dstEndRule>ESPIB40E2000</dstEndRule>ESPI

… to be placed into a <LocalTimeParameters>(from ESPI) element.

<feed ... >
    ...
    <!-- LocalTimeParameters -->
    <entry>
        ...
        <title>DST For Eastern Time in North America</title>
        <content>
            <espi:LocalTimeParameters>
                <espi:dstEndRule>B40E2000</espi:dstEndRule>
                <espi:dstOffset>3600</espi:dstOffset>
                <espi:dstStartRule>360E2000</espi:dstStartRule>
                <espi:tzOffset>-18000</espi:tzOffset>
            </espi:LocalTimeParameters>
        </content>
    </entry>
    ...
</feed>

A rule value of <dstStartRule>ESPIFFFFFFFF</dstStartRule>ESPI or <dstEndRule>ESPIFFFFFFFF</dstEndRule>ESPI means rule processing/DST correction is disabled.

Another DST End Example:

In the European Union, DST currently starts on the last Sunday of March and ends on the last Sunday of October.  The change is at 1:00 AM UTC (01:00), so it differs by time zones in Europe: e.g., 02:00 local time in Central Europe (CET) and 03:00 local time in Eastern Europe (EET).

Using Finland (EET) as an example of when DST ends: the rule is…

Month: 10 (“October”)
Operator: 7 (“the last occurrence of the Day of the Week in a month”)
Day of Month: 0 (“not applicable”)
Day of Week: 7 (“Sunday”)
Hours: 3 (“3:__”)
Seconds: 0 ( “__:00” or 00 minutes)

Then, the hexadecimal is used in the dstEndRule:

<dstEndRule>ESPIAE0E3000</dstEndRule>ESPI

… to be placed into a <LocalTimeParameters>(from ESPI) element.

Time Zone Offset

The time-zone offset found in the LocalTimeParameters element is used by the Third Party to adjust the UTC date-time of each related timestamp.  It allows the Third Party to correlate interval readings with local times of day for their clients.

tzOffset is an offset from Standard Time (not Daylight Savings Time) in a value of seconds; either positive or negative.

For example, in the North American Eastern Standard Time Zone, the offset from UTC is currently recognized by the USA to be -05:00 (or in US law as, "the standard time of the first zone shall be Coordinated Universal Time retarded by 4 hours; that of the second zone retarded by 5 hours […]"¹).  Similarly, Ontario, Canada states this as, "five hours behind Greenwich time."²  Canada defines time-zone rules by province.

An example, therefore, of a -05:00 offset in the ESPI standard is:

<tzOffset>ESPI-18000</tzOffset>ESPI

If DST in active for a given interval (based on the parameters of the dstStartRule and dstEndRule), then the offset is adjusted by the Third Party by the number of seconds found in the dstOffset:

For example:

<dstOffset>ESPI3600</dstOffset>ESPI and <tzOffset>ESPI-18000</tzOffset>ESPI

… would result in a time-zone offset of -04:00 instead of -05:00 for the given interval(s) in the DST date range.