It's about TIME David Hoppe @hopasaurus hopasaurus@gmail.com
It's time, so what!
What is Time? What is a day? What is an hour? What is a minute? What is a second?
Lets start at the beginning... What is a Day?
How is time measured? By the day Sundials Mechanical Clocks Electronic Clocks
Things get weird! ● We start to notice things. ● Hours are not uniform between the seasons – Sundials and mechanical clocks don't always agree ● Days are not always the same length – As clock get more precise we notice accuracy slipping
The equation of time "Tijdvereffening-equation of time-en" by Clem Rutter, Rochester, Kent. - Originally from nl.wikipedia; description page is (was) here * 7 jan ● 2005 23:57 Willy Leenders 549x375 (49.479 bytes) (grafiek tijdsvereffening, eigen creatie)Modifications 2011:Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Tijdvereffening-equation_of_time- en.jpg#mediaviewer/File:Tijdvereffening-equation_of_time-en.jpg
Which one is “right”?
Time “Standards” ● Atomic Time (TAI) ● Universal Time (UT) – UT0, UT1 ● Coordinate Universal Time (UTC) ● Dynamical Time ● Geocentric Coordinated Time ● Barycentric Coordinated Time ● Sidereal Time
Leap Second? INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE REFERENCE SERVICE DE LA ROTATION TERRESTRE DE L'IERS OBSERVATOIRE DE PARIS 61, Av. de l'Observatoire 75014 PARIS (France) Tel. : 33 (0) 1 40 51 22 26 FAX : 33 (0) 1 40 51 22 91 e-mail : services.iers@obspm.fr http://hpiers.obspm.fr/eop-pc Paris, 5 January 2015 Bulletin C 49 To authorities responsible for the measurement and distribution of time UTC TIME STEP on the 1st of July 2015 A positive leap second will be introduced at the end of June 2015. The sequence of dates of the UTC second markers will be: 2015 June 30, 23h 59m 59s 2015 June 30, 23h 59m 60s 2015 July 1, 0h 0m 0s The difference between UTC and the International Atomic Time TAI is: from 2012 July 1, 0h UTC, to 2015 July 1 0h UTC : UTC-TAI = - 35s from 2015 July 1, 0h UTC, until further notice : UTC-TAI = - 36s
Why do we need leap seconds?
How does all this affect me?
"QSL card sent to listener confirming reception of WWV from Maryland - 194007" by http://tf.nist.gov/images/radiostations/wwv40.jpg;. Licensed under ● Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:QSL_card_sent_to_listener_confirming_reception_of_WWV_from_Maryland_- _194007.jpg#mediaviewer/File:QSL_card_sent_to_listener_confirming_reception_of_WWV_from_Maryland_-_194007.jpg
NTP and friends ● Automatic time synchronization between hosts. ● Difficult to agree on inserting leap seconds ● More difficult to remove them
Time Bugs
ZUNE 30 ● December 31 st , 2008 lockup ● Finger pointing ● Toshiba did it too (ever heard of Gigabeat S?) ● It was Freescale's fault ● Just fudge the clock
Excel Bug ● Excel pretends that February 29, 1900 exists ● It doesn't ● Why? ● Lotus 1-2-3 started it.
Y2K ● Lets just use the last two digits for the year ● It will save space ● What could possibly go wrong?
Y2.038K ● Cause: using a 32bit signed int for storing time ● AOL learned early ● Some have yet to learn ● Really, you didn't learn from Y2K???
Tips for dealing with date/time: ● Don't write your own parser ● Don't do your own math ● Be careful with local time vs. UTC Time ● Know how the time is being stored – Will it work until you retire? – When is the “end” of time? ● Know how much precision/accuracy is needed. ● Wrap access of the time from the system.
Testing Time ● Don't change the time on your server... ● Check Leap year – February 29 and December 31 ● Check non-Leap year ● What will happen on a leap second? ● What happens with an invalid date? – February 30, 2015? ● Don't hard code dates into your tests
How should a Date be written? http://xkcd.com/1179/
Fun facts ● Watch crystals ● Metric time ● How to tell a real Rolex from a fake? ● Who is in charge of time?
End of Time?
What time is it now?
It's about TIME David Hoppe @hopasaurus hopasaurus@gmail.com
It's time, so what! A while back I was working on a project involving Receiving GPS signals and forwarding the data to a server. GPS signals contain the current time. AND it is a very accurate representation of the current time. HOWEVER when the signals got to the server the time was reported to be IN THE FUTURE!
What is Time? What is a day? What is an hour? What is a minute? What is a second? In Book 11 of St. Augustine's Confessions, he ruminates on the nature of time, asking, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not. " A second was:1/86400th of a day The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
Lets start at the beginning... What is a Day? . Do the siderial time exercise. This will start to give a sense that all is not as it seems.
How is time measured? By the day Sundials Mechanical Clocks Electronic Clocks
Things get weird! ● We start to notice things. ● Hours are not uniform between the seasons – Sundials and mechanical clocks don't always agree ● Days are not always the same length – As clock get more precise we notice accuracy slipping We start to notice things Hours are not uniform Days are not always the same length.
The equation of time "Tijdvereffening-equation of time-en" by Clem Rutter, Rochester, Kent. - Originally from nl.wikipedia; description page is (was) here * 7 jan ● 2005 23:57 Willy Leenders 549x375 (49.479 bytes) (grafiek tijdsvereffening, eigen creatie)Modifications 2011:Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Tijdvereffening-equation_of_time- en.jpg#mediaviewer/File:Tijdvereffening-equation_of_time-en.jpg http://en.wikipedia.org/wiki/Equation_of_time So people figured out that the sun dial and the mechanical clock wouldn't agree all the time so they cracked open their history books and looked up this “ptolemy” guy who did a lot of math in ancient egypt and figured this all out for them already. This is partially due to the ecentricity of Earth's orbit also partially due to the Obliquity of the elipctic (pretty sure we humans refer to this as the tilt of the axis) The total variation from one extreme to the other ends up being about half an hour.... whatever an hour is. So now we have two time “standards” Solar time and Mechanical time. Or Apparent time and Mean time.
Which one is “right”?
Time “Standards” ● Atomic Time (TAI) ● Universal Time (UT) – UT0, UT1 ● Coordinate Universal Time (UTC) ● Dynamical Time ● Geocentric Coordinated Time ● Barycentric Coordinated Time ● Sidereal Time Atomic Time (TAI) – International Atomic Timescale based on a “large number” of atomic clocks Universal Time (UT) is counted from 0 hours at midnight with the unit of duration being the mean solar day (a solar day adjusted to be as uniform as possible dispite variations in Earth's rotation) UT0 and UT1 are related. Coordinated Universal Time (UTC) differs from TAI by an integral number of seconds. UTC is kept within 0.9 seconds of UT through the introduction of leap seconds. So far all leap seconds have been positive. UTC has replaced “GMT” as a standard. Dynamical Time, Geocentric Coordinated Time (TCG), Barycentric Coordinated Time (TCB), Sidereal Time (this one is interesting) Show the graph of “Variablility of Earth's rotation” And these are just the “Scientific” standards.
Leap Second? INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE REFERENCE SERVICE DE LA ROTATION TERRESTRE DE L'IERS OBSERVATOIRE DE PARIS 61, Av. de l'Observatoire 75014 PARIS (France) Tel. : 33 (0) 1 40 51 22 26 FAX : 33 (0) 1 40 51 22 91 e-mail : services.iers@obspm.fr http://hpiers.obspm.fr/eop-pc Paris, 5 January 2015 Bulletin C 49 To authorities responsible for the measurement and distribution of time UTC TIME STEP on the 1st of July 2015 A positive leap second will be introduced at the end of June 2015. The sequence of dates of the UTC second markers will be: 2015 June 30, 23h 59m 59s 2015 June 30, 23h 59m 60s 2015 July 1, 0h 0m 0s The difference between UTC and the International Atomic Time TAI is: from 2012 July 1, 0h UTC, to 2015 July 1 0h UTC : UTC-TAI = - 35s from 2015 July 1, 0h UTC, until further notice : UTC-TAI = - 36s https://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat Yup leap second, it is Bulletin C put out by the IERS... the WHAT? IERS. International Earth Rotation Service. They keep the globe spinning.
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