Time Scales and Time Zones Jim Ingleson NYISO [email protected] 2006 FDA Conference GA Tech May 1 – 2, 2006.

Time Scales and Time Zones

Jim InglesonNYISO

[email protected] FDA Conference

GA Tech May 1 – 2, 2006

13:47:04.563 UTC

• If you were presented with the above time, please consider what it implies to you (now).

Time Scales• Many Time Scales• Recognized Standard Time

Scales• Consider “Town Hall Time”• Stability• Availability• Rules• Correction to solar

observation, correction by monitoring another time scale.

• “Steering”

Town Hall Time Scale

• Someone could synchronize a watch to “town hall time” and consider that they are carrying town hall time, accurate enough for their purpose.

• Where would you go to pick up “town hall time?”

Time Scales in General

• Times scales, particularly widely recognized time scales, include a good clock or clocks, various rules, methods of distribution. Notice the important distinction between “time scale” and “clock.”

• Many time scales are “astronomical” such as UT, GMT (both no longer used), UT0, UT1, and UT2.

An “Atomic” Time Scale

• In 1967 the second became an SI unit.

• Our “standard” time scale is now based on precise measurement of this defined second.

The TAI & UTC Time Scales

• TAI is based on the absolute second as measured using cesium clocks in many world laboratories and coordinated by BIPM (France).

• UTC is also maintained by these laboratories. “Leap seconds” are added to UTC when necessary to keep the difference between UTC and UT1 to less than 0.9 s. 23 leap seconds have been added thus far. When necessary, leap seconds are always added at the end of December or June. UTC is sometimes called a “paper” time scale.

• Observing a leap second: … 58, 59, 60, 0, 1, 2, …

Conversion Factors

• Remember that an extra second occurred at the end of the year 2005 and certainly will occur again.

• The last minute of 2005 had 61 seconds.• The last hours of 2005 had 3601 seconds.• The last day of 2005 had 86401 seconds.• 2005 (not a leap year) had 31536001 seconds.• By the way 2004 and 2008 are years which

contain a leap day.

No clock is perfect

• Many Cesium clocks in laboratories around the world very carefully coordinated with each other are not perfect, either.

• UTC(NIST) a a particular “realization” of UTC. NIST is one of the participants, or of the 50 (80) laboratories.

UTC(BIPM)

UTC(NIST)

UTC(USNO)

80 Others

(20 ns)

(Many other means of disseminating UTC.)

UTC(USNO)

GPS System

GPS Receiver

(20 ns)

Now 14 seconds ahead of UTC

(Corrections Applied)

GPS Receiver

What comes out of a GPS receiver is very nearly UTC. Uncertainty can be on order of 1 microsecond. For our purposes, we consider it to be UTC, but it is not truly UTC.

UTC Time Scale

• UTC is really the “only game in town.” Probably no other time scale is worthy of consideration, except for TAI.

• Now, consider that UTC is often presented “raw” with no offset, but it can also be presented with whatever offset is desired, EDT for example is 4 hours behind “raw” or zero offset.

Time Zones

Offset Letter Also Called

-8U Uniform Pacific Standard Time (PST)

-7T Tango Mountain Standard Time (MST)

-6S SierraCentral Standard Time (CST)

-5R Romeo Eastern Standard Time (EST)

-4Q Quebec Atlantic Standard Time (AST)

-3P Papa Greenland Standard Time (GST)

-2O Oscar Greenland Eastern Standard Time

(VTZ)

-1N November Azores Time (AT)

0Z Zulu Western Europe Time (WET)Unfortunately, the last time zone is also called by the names of several time scales, i.e. GMT and UT (no longer used), and UTC, which is currently our main worldwide standard time scale.

“Zulu” Time Zone

• To change EDT to Zulu, add 4 hours.• To change EST to Zulu, add 5 hours.• Example: At 0830 EDT today, it is of course

0730 EST. The Zulu time at the same hour is 1230, 30 minutes after noon.

• Some way to get mentally used to Zulu time.

What shall we call the time zone with no offset?

• Z, Zed, or Zulu?

• WET?

• UT?

• GMT?

• UTC?

• UTC Time Zone?

• UTZ?

Terminology Problem

• Unfortunately, the Zulu time zone is also called by the names of several time scales, i.e. GMT and UT (no longer used), and UTC, which is currently our main worldwide standard time scale.

• This is not a problem from the point of view of the NIST staff. When NIST announces a time in UTC, that time is a correct representation of both UTC time scale and UTC time zone.

UTC

• Although UTC is definitely not a Time Zone, when the letters “UTC” are presented immediately after a time, I suggest that we understand that to mean UTC is presented “RAW” or “straight,” or in other words, there is no time zone offset applied.

Daylight Saving Time

• The (USA) Uniform Time Act of 1966 does not require that DST be observed, but if DST is observed, it must the done uniformly. For example, DST is not observed in the Eastern Time Zone part of Indiana.

• Ammended in 1986.• Energy Policy Act of 2005.

Daylight Saving Time

• Spring Forward date will change:

• 2006 - First Sunday in April

• 2007 - Second Sunday in March

• Fall Back date will change:

• 2006 – Last Sunday in October

• 2007 – First Sunday in November

• Could this policy be changed in the future?

Many GPS Clocks are Programmed with the old dates!

• What are the possible solutions to this problem?

• Number 1 possible solution is to change the clock IRIG-B output stream to Zulu time zone, a.k.a. UTZ time zone.

• An example of the directions for a particular example clock follow…….

- Press the SETUP key repeatedly until the display says "Set IRIG Time Data?"

- Press ENTER key. The display will say "Select UTC or LCL."

- Use the DOWN key to change to "IRIG DATA TIME = UTC"

- Press ENTER key to confirm choice and move on. The display will now say "IRIG-B IEEE 1344? - 1344 OFF"

- Exit the setup mode by pressing some other key such as TIME.

Solution Number 1

• You can do this one anytime between now and March 11, 2007.

• Above is the date of the first change with the new date, which you clock is not programmed for.

• Never again worry about changeovers.• Not necessary to make any physical change

to the clock itself.

Solution Number 2

• The clock manufacturer’s instructions and advice should be followed, but this information is presented as an example.

• Sometime after October 29, 2006, visit the clock and …………….

Convert Clock from Pre-programmed to User-Programmed

• You will need to wait until after Fall 2006 changeover to standard.

• Clock case must be opened and EPROM changed.• Considerable front panel programming will be

required in most cases.• The following is an example taken from a

prominent manufacturer’s website as an example. (Edited only to remove name and model numbers.)

The tentative plan is to change the DST selections to OFF, ON, and AUTO. AUTO will become a manually configurable date that is similar to the existing AUTO-CUSTOM feature. We will ship clocks with AUTO configured for the existing DST settings. After October 2006, you will have to configure the new DST settings. This configuration will only have to be completed one time unless DST changes again.

This solution will involve a firmware ROM replacement. Some older GPS clocks may also require a receiver upgrade. It may be best to wait until after October 29, 2006 to upgrade the GPS clocks that only require the firmware replacement. That way the chip change and new DST settings can be completed in one trip to the substation.

All clocks have two DST work-arounds available. The first is to manually change the local offset and the second is to manually turn the DST feature on/off.

WAIT!

Just say “NO.”

Begin now to convert to UTZ!

• No delay is necessary

• Any future confusion from DST eliminated

• We will never achieve full conversion to UTZ.

Please consider now what is implied by the following times:

• 13:47:04.563 Z• 13:47:04.563 UTZ• 13:47:04.563 GMT, or UT• 13:47:04.563 UTC• 09:47:04.563 EDT• 2005 December 31

23:59:60.563

Conclusions

• When “UTC” appears after a time, don’t assume it implies synchronization to the UTC time scale.

• In fact, any letters after an expression of time should be taken as a time zone designation.

• We intend to report all times closely synchronized to UTC time, accurate enough for the purpose.

• UTC is most conveniently available in the output of a GPS clock.

• Keep leap seconds in the back of your mind.• We suggest the terms “GPS Time” or “GPS Time Scale”

not be used.

Conclusions

• “UTC” is a poor time zone designation because it is the name of our principal worldwide time scale, however many do use it, and we have to accept it.

• GMT, UT, are sometimes used; they are the name of old time scales, no longer maintained, and as such are better than UTC, less likely to cause confusion.

• We suggest that “Z” and “UTZ” or “Universal Time Zone” are good time zone terminology.

• Switch to UTZ now!

Time Scales and Time Zones

Jim InglesonNYISO

[email protected] FDA Conference

GA Tech May 1 – 2, 2006