20151022 time and computers softshake -...
TRANSCRIPT
TimeandComputersNicolasSeriot2015-10-22
Amanwithawatchknowswhat.meitis.Amanwithtwowatchesisneversure.
Unesecondeaprèsle2015-06-3023:59,iln'étaitpas2015-07-0100:00mais2015-06-3023:60.Eneffet,leBureauInterna.onaldesPoidsetMesures(BIPM)a,commepresquechaqueannée,décidéd'ajouterunesecondeintercalaireautempscivilinterna.onal(UTC)pourtenircompteduralen.ssementdelarota.ondelaterre.
L'ajoutdeceSesecondeposed'importantsproblèmesdeprogramma.onetdesynchronisa.on.Plusieursplacesdeboursesuspendentainsileséchangesautourdelasecondefa.dique.CeSesecondeintercalaireestpassablementcontestée,aupointquel'UnionInterna0onaldesTélécommunica0on(UIT)seréuniraennovembreàGenèvepourdéciderounondesasuppression.Sitelestlecas,d'autresproblèmesrisquentalorsdesurvenir.Dansquelquessiècles,ilpourraitfairenuitàmidi!
CeSeréuniondel'UITestl'occasiond'examinercommentlessystèmesinforma.quesgèrentceSesecondeintercalaire,commentlesordinateurssynchronisentleurshorloges(NTP),quellessontleshorlogesderéférences,commentsontgéréslesfuseauxhorairesetl'heured'été,quelleserreursdeprogramma0onéviter(dis.nc.ondateetdurée),etc.Lesexemplesserontbaséssurl'environnementUnix/OSX.
TimekeepingHistory
GMT/UT-UniversalTime
• 19thcentury:railandtelegraphyimpliedneedtosynchroniseFmeinternaFonally
• 1884:GreenwichMeanTime(GMT)adoptedastheworld’sstandardFme,24Fmezones
• BasedonmeansolarFme,86400seconds/day
• Adoptedbyallcountriesbetween1848-1972
• Atomicalclocksallowsub-secondprecision
• ScienFficandindustrialneeds,telecom,…
1960s
http://homepages.abdn.ac.uk/npmuseum/article/Astroimages/16image.jpg
SISecond(Intl.SystemofUnits)
Thesecondisthedura.onof9192631770periodsoftheradia.oncorrespondingtothetransi.onbetweenthetwohyperfinelevelsofthegroundstateofthecaesium133atom.
(13thConférenceGénéraledesPoidsetMesures,1967)
…atsealevel(bewarerelaFvity…).
…referstoacaesiumatomatrestatatemperatureof0K.(1997)
OtherphysicalunitssuchasampereandmeterrelyonthisaspartoftheirowndefiniFons.
Themeteristhelengthofthepathtravelledbylightinvacuumduringa.meintervalof1/299792458ofasecond.
(17thConférenceGénéraledesPoidsetMesures,1983)
ImportanceofSecondDefiniFon
AtomicclockFOCS1,FederalOfficeofMetrology,Bern,SwitzerlandAsof2004,offby1sin30millionyears
AtomicclockNIST-F2,Boulder,Colorado,USAAsof2014,offby1sin300millionyears
OpFcalLaaceClock
Nature,March2015
SystemaFcevaluaFonofanatomicclockat2×10−18totaluncertainty
Themostpreciseatomicclockyet,usingstronFumatoms,keepsFmeaccuratelytowithinasecondper15billionyears.
TAI–InternaFonalAtomicTime• StaFsFcalFmescalebasedon300+atomicclocksin60+labsworldwide.
• ContributorsinBIPMCircular-T,updatedmonthly.
• MoststableclocksreceivethemostweightinthecalculaFon.
• Timeisknownonlyaposteriori,afermeasurement.
CIRCULAR T 333 ISSN 1143-1393 2015 OCTOBER 12, 14h UTC BUREAU INTERNATIONAL DES POIDS ET MESURES ORGANISATION INTERGOUVERNEMENTALE DE LA CONVENTION DU METRE PAVILLON DE BRETEUIL F-92312 SEVRES CEDEX TEL. +33 1 45 07 70 70 FAX. +33 1 45 34 20 21 [email protected] 1 - Coordinated Universal Time UTC and its local realizations UTC(k). Computed values of [UTC-UTC(k)] and uncertainties valid for the period of this Circular. From 2015 July 1, 0h UTC, TAI-UTC = 36 s. Date 2015 0h UTC AUG 30 SEP 4 SEP 9 SEP 14 SEP 19 SEP 24 SEP 29 Uncertainty/ns Notes MJD 57264 57269 57274 57279 57284 57289 57294 uA uB u Laboratory k [UTC-UTC(k)]/ns AOS (Borowiec) -0.3 -1.0 -1.5 -1.8 -1.5 0.4 1.1 0.3 5.0 5.1 APL (Laurel) -4.8 -2.6 -1.1 2.8 4.3 4.8 4.1 0.3 4.9 4.9 AUS (Sydney) -675.5 -647.2 -633.5 -614.1 -597.8 -581.3 -558.3 0.3 5.0 5.1 BEV (Wien) 0.8 -1.8 -8.4 -18.0 -28.6 -22.2 -11.3 0.3 3.1 3.1 BIM (Sofiya) 2693.2 2725.9 2765.9 2780.0 2784.9 2821.8 2847.7 1.5 7.0 7.2 BIRM (Beijing) - -97.6 -104.8 -118.2 -117.7 -119.1 -134.7 1.5 20.0 20.1 BY (Minsk) 4.3 4.2 3.7 5.8 4.5 8.1 8.1 1.5 7.0 7.2 CAO (Cagliari) -7595.2 -7696.5 -7797.3 -7898.8 -7988.6 -8084.2 -8188.0 8.0 7.0 10.7 CH (Bern-Wabern) 12.3 14.0 12.5 11.3 10.8 12.1 6.8 0.3 1.2 1.3 CNM (Queretaro) -1.9 -2.7 -1.4 -1.1 0.3 -1.0 -1.8 3.0 5.0 5.8 . . .2 - International Atomic Time TAI and Local atomic time scales TA(k). Computed values of [TAI-TA(k)]. 3 - Difference between the normalized frequencies of EAL (free atomic time scale) and TAI. 4 - Duration of the TAI scale interval. 5 - Relations of UTC and TAI with predictions of UTC(k) disseminated by GNSS and their System Times. 6 - Time links used for the computation of TAI and their uncertainties.
USOfficialTime
hgp://tycho.usno.navy.mil/mc_to.html
UTC(NIST)andUTC(UNSO)bothkeeptheiratomicclocksnetworkandtheirownversionofUTC,inverycloseagreement(about20ns).TheycontributetoInternaFonalAtomicTime(TAI)andCoordinatedUniversalTime(UTC).
EarthRotaFon• SIsecondbasedonEphemerisTimescalemeasuredin19thcentury,slightlyshorterthanthemeansolarsecond
• Earth'srotaBontendstobeslowing,becauseofFdes,climaFcandgeologicalevents.
• MeansolarBmedayingraduallyincreasing before1820:<86400seconds ~1820:86400seconds 2015:about86400.002seconds
CurrentlytheEarthrunsslowatroughly2millisecondsperday.Afer500days,thedifferencebetweenUT1andTAIwouldbe1second.
hgp://tycho.usno.navy.mil/leapsec.html
UTC-CoordinatedUniversalTime• Acronymstandsfor«CoordinatedUniversalTime»and«tempsuniverselcoordonné»nottofavoranylanguage.
• Createdinthe1950stodisplaythesameFmeworldwideandcoordinateFmeradiobroadcaststhroughtheworld.
• TimeissynchronisedthankstocoordinatedshiHstomatchUT.
ephemerissecond+customsteps1second=1/31556925.9747ofthetropicalyear1900January0at12hoursephemeris.me
atomicsecond(SI) 1stJanuary1972:reset UTC00:00:00==1January197200:00:10TAI
UTCnowFcksinSIseconds,instepwithTAI.
Keptwithin0.9secondsofUT1withleapseconds.
UTC=TAI+int.nbofseconds,currently36
DifferentSecondDefiniBons,IrregularShiHs
DUT1CorrecFon
hgp://www.nist.gov/pml/div688/grp50/leapsecond.cfm
LeapSeconds• InternaFonalEarthRotaFonandReferenceSystemsServiceakaIERSaddsorremoves1secondtoUTCwheneverneededtokeepUTCwithinUT1+/-0.9s.
• IERSmonitorstheEarthrotaFon,calculatesandpublishesUT1inIERSBulleFnB.
• BulleFnBisusedfordecidingwhentoaddaleapsecondtoUTC,publishedinBulleFnC.
• Currentlyaboutevery18months,atendofDecemberorJune,orelseendofMarchorSeptember(neverhappened).
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
hgps://hpiers.obspm.fr/iers/bul/bulc/bulleFnc.49
LeapSecondsDebates• Withoutleapseconds,noonontheclockwillbemidnightintheskysome3,000yearsfromnow(unlessweadjustFmezones)
• Announced6+monthsinadvance,butnotallsourcesuseit(eg.GermanradiosignalDCF77doesannounceleapsecondsonlyonehouraheadofFme)
• Notaviablelong-termsoluBonNextcentury:aleapsecondwillberequiredeveryyear2600:3or4leapsecondswillberequiredeachyear4300:addleapsecondseverymonth
StockExchanges
hgp://www.nasdaq.com/arFcle/markets-are-jumpy-over-coming-leap-second-20150518-01174
hgp://www.cmegroup.com/company/leap-second.html
«Toaccountfortheleapsecond,anextrasecondthatwillbeaddedtoCoordinatedUniversalTime(UTC)onJune30,2015,CMEGroupisdelayingtheCMEGlobexmarketopenfortradedateJuly1.»
«LargeU.S.stockexchangesincludingNewYorkStockExchange'sNYSEArca,NasdaqOMXGroupInc.'sNasdaqandBATSGlobalMarketswillclosetheirafer-hourssessions30minutesbeforetheirregular8p.m.close.»
UTCCalculaFons• ReconstrucBngpastisamess
• NumberofSIsecondselapsedsinceanydatebeforeatomicFme?Noway!
• Nomberofsecondslegallyellapsedsincesomedateinthepast?DependsonjuridicFons! hgp://www.ucolick.org/~sla/leapsecs/epochFme.html
• Futureisimpossibletotellforsure
• NumberofSIsecondsbeforesomefutureUTCdate?Noway!
DebatesinITU-R
• Since2001,ITU-Rhasbeenconsideringsuppressingleapseconds.
• In2015NovemberITU-RWRC-15willagaindiscussleapsecondsinUTC.
POSIXTime• Easytodealwithformachines,nomonthsetc,basicallyasimplecounter
• POSIXFmestampactuallycountsthenumberofUTsecondssince1970-01-01
• Eachandeverydayisexactly86400seconds(POSIX.1-2008,4.15)
• Timezoneinanenvironmentvariablehgp://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html
• gettimeofday()obtainssecondsandmicrosecondssinceJanuary1,197000:00UTC.hgp://pubs.opengroup.org/onlinepubs/007908775/nottobeusedformeasuringBmeintervals!
MeasuringTimeIntervals1445472703 - 157187 - 02:11:431445472704 - 157188 - 02:11:441445472705 - 157189 - 02:11:451445469104 - 157190 - 01:11:44 # time changes1445469105 - 157191 - 01:11:451445469106 - 157192 - 01:11:46
POSIXFmestamp
monotonousfuncFonmach_absolute_time() orCACurrentMediaTime(),equivalenttoLinuxclock_gettime() nbofsecondselapsedsinceunspecifiedevent,eg.reboot
POSIXandLeapSeconds• Leapsecondsarejustignored
• AlignmentwiththeactualFmeisimplementaFondefined(POSIX.1-2008,4.15)
• PosiFveleapsecondshavenoUNIXFmestamprepresentaFon
UTC - timeIntervalSince1970
2015-06-30 23:59:59 - 14357087992015-06-30 23:59:60 - 1435708800 <- unknown2015-07-01 00:00:00 - 14357088002015-07-01 00:00:01 - 1435708801
NSDate *dateFromString(NSString *s) { NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; NSLocale *posixLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]; [dateFormatter setLocale:posixLocale]; [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"]; [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]]; return [dateFormatter dateFromString:s]; }
/* Cocoa can't build date with leap second */
NSDate *date1 = dateFromString(@"2015-06-30 23:59:59"); NSLog(@"-- %@", date1); // 2015-06-30 23:59:59 +0000
NSDate *date2 = dateFromString(@"2015-06-30 23:59:60"); NSLog(@"-- %@", date2); // null
/* NSDate computations just ignores leap seconds */
NSDate *date3 = dateFromString(@"2015-01-01 00:00:00"); NSDate *date4 = dateFromString(@"2016-01-01 00:00:00"); NSLog(@"-- %f", [date4 timeIntervalSinceDate:date3]); // 31536000 = 365 * 24 * 60 * 60
void printTimestampAndDate(const char* s) { struct tm tm; memset(&tm, 0, sizeof(struct tm)); strptime_l(s, "%Y-%m-%d %H:%M:%S", &tm, NULL);
time_t time = mktime(&tm);
char buf[255]; strftime(buf, sizeof(buf), "%d %b %Y %H:%M:%S", &tm); printf("-- %ld \t %s\n", time, buf); NSDate *date = [NSDate dateWithTimeIntervalSince1970:time]; NSLog(@"-- %@", date); }
/* time.h functions can represent leap second but same as next second */
printTimestampAndDate("2015-06-30 23:59:59"); // -- 1435705199 01 Jul 2015 00:59:59 printTimestampAndDate("2015-06-30 23:59:60"); // -- 1435705200 01 Jul 2015 01:00:00 printTimestampAndDate("2015-07-01 00:00:00"); // -- 1435705200 01 Jul 2015 01:00:00
• POSIXwasdesignedbeforetheintroducBonofleapseconds.
• Duringleapseconds,time_tcannotrepresentUTCduringleapsecondsnon-ambiguously.
• POSIXbasicallypretendstocount86400SIsecondsperday,whichisnotpossible.
• MiFgaFon:don’tstoreUNIXbinaryBmestampsbutUTCdateFmeinstead.
POSIXDefiniFonisInconsistent
POSIX32bitsoverflow
$ date -ur `echo $((2**31-1))`Tue Jan 19 03:14:07 UTC 2038
32bitstime_twilloverflowafer2038-01-19T03:14:07UTC
GPS-GlobalPosiFoningSystem• Eachsatellitehasat2+onboardcaesiumatomicclocksand 2+rubidiumatomicclocksasback-up.
• TimemeasuredfromsatellitesisgatheredbymonitorstaBons,processedinColoradoanduploadedagain.
• PeriodiccorrecBonsareperformedontheon-boardclockstocorrectrelaFvisFceffectsandkeepthemsynchronizedwithgroundclocks.
• GPSreceivercomparessignalsfrom4satellites,computeslocaFononearthbyanalysingFmingdelays.
Vandenberg AFB California
Alternate Master Control Station
Air Force Monitor Station
Hawaii
Master Control Station
Schriever AFB Colorado
NGA Monitor Station
South Korea
Australia
Bahrain
South Africa
United Kingdom
Argentina
Ecuador
USNO Washington
Alaska
New Zealand
AFSCN Remote Tracking Station
Ascension Diego Garcia
Cape Canaveral Florida
Kwajalein
Ground Antenna
New Hampshire
Greenland
Guam
Updated April 2014
GPS Control Segment
GPSandLeapSeconds• SinceGPSEpochJanuary6,1980,GPSBme==TAI+19seconds
• AnUTCcorrecBonisprovidedaspartofthebroadcastmessage(currentlyUTC+17leapsecondssince1980)
• DeltaGPS/UTCissenton8bits,sothecounterwillprobablyoverflowaround2060-2090dependingonthenumberofleapseconds.
NTP-NetworkTimeProtocol• InventedanddeveloppedbyProf.DavidMillsfirstRFCfrom1988
• NTP(RFC-5905,port123)isthemostcommonlyusedBmeprotocol.SupersedesDayFmeProtocol(port13)andTimeProtocol(port37).
• TheclientperiodicallypollsBmeservers,ignoresresponsesfromserversthatappeartobesendingthewrongFme,andaveragestheresultsfromthosethatappeartobecorrect.
• Mostdesktopclientsmakeasinglerequesttosetthecomputer’sclock.TheyareactuallySNTP(SimpleNetworkTimeProtocol)clients.
SNTP–SimpleNetworkTimeProtocol
$ sntp time.euro.apple.com2015 Jul 07 22:44:26.122392 +6.184477 +/- 0.001022 secs
# fix local time by adding +6.184477 seconds# time is believed to be correct +/- 0.001022 secs
$ sudo sntp -s ch.pool.ntp.org # set time
$ sudo sntp -j time.nist.gov # adjtime
ADJTIME(2) BSD System Calls Manual ADJTIME(2)
NAME adjtime -- correct the time to allow synchronization of the system clock
SYNOPSIS #include <sys/time.h>
int adjtime(const struct timeval *delta, struct timeval *olddelta);
DESCRIPTION Adjtime() makes small adjustments to the system time, as returned by gettimeofday(2), advancing or retarding it by the time specified by the timeval delta. If delta is negative, the clock is slowed down by incre- menting it more slowly than normal until the correction is complete. If delta is positive, a larger increment than normal is used. The skew used to perform the correction is generally a fraction of one percent. Thus, the time is always a monotonically increasing function. A time correc- tion from an earlier call to adjtime() may not be finished when adjtime() is called again. If olddelta is non-nil, the structure pointed to will contain, upon return, the number of microseconds still to be corrected from the earlier call.
NTPandLeapSecondsRFC 958
5.4. Leap Seconds
A standard mechanism to effect leap-second correction is not a part of this specification. It is expected that the Leap Indicator bits would be set by hand in the primary reference clocks, then trickle down to all other clocks in the network, which would execute the correction at the specified time and reset the bits.
AsPOSIX,theNTPcounterignoresleapseconds. Since1972,NTPFcksinSIseconds,withUTCandTAI.
Butitaccumulatesmeansolarseconds.
LeapSecond–Jumps1435708799 Tue Jun 30 23:59:59 UTC 20151435708799.5 Tue Jun 30 23:59:59.5 UTC 20151435708800 Tue Jun 30 23:59:60 UTC 2015 # leap second1435708800.5 Tue Jun 30 23:59:60.5 UTC 2015 # leap second1435708800 Wed Jul 1 00:00:00 UTC 20151435708800.5 Wed Jul 1 00:00:00.5 UTC 20151435708801 Wed Jul 1 00:00:01 UTC 20151435708801.5 Wed Jul 1 00:00:01.5 UTC 2015
Bad:youdon’tknowwhichFmestampcomesfirstduringleapsecond.Bad:youcan’tmeasureFmeaccuratelyduringleapsecondsBad:wriFngafileeachsecondnamedaccordingtoFmestampwouldoverwritedataBad:issuealsoexistewhensystemclockischangedaferbeingoutofsync
LeapSecond–Smearing1435708799 Tue Jun 30 23:59:59 UTC 20151435708799.5 Tue Jun 30 23:59:59.5 UTC 20151435708800 Tue Jun 30 23:59:60 UTC 2015 # leap second1435708800.25 Tue Jun 30 23:59:60.5 UTC 2015 # leap second1435708800.50 Wed Jul 1 00:00:00 UTC 20151435708800.75 Wed Jul 1 00:00:00.5 UTC 20151435708801 Wed Jul 1 00:00:01 UTC 20151435708801.5 Wed Jul 1 00:00:01.5 UTC 2015
Good:Fmeisstretchedsothatgettimeofday()iskeptstrictlymonotonousBad:thisyieldsneitherSIsecondsnorUTCFme.NouniversalwaytosmearFme.Google’swayhgps://googleblog.blogspot.ch/2011/09/Fme-technology-and-leaping-seconds.html
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|LI | VN |Mode | Stratum | Poll | Precision |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Root Delay |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Root Dispersion |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Reference ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Reference Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Origin Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Receive Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Transmit Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|0 1|1 0 0|0 1 1| Stratum | Poll | Precision |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Root Delay |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Root Dispersion |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Reference ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Reference Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Origin Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Receive Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| |+ Transmit Timestamp (64) +| |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ATiny,79charactersNTPClient
01100011 = 0x63 = ‘c’
$ date -r$((0x`printf c%47s|nc -uw1 ntp.metas.ch 123|xxd -s40 -l4 -p`-64#23GDW0))Thu Oct 22 01:24:34 CEST 2015
hgp://seriot.ch/ntp.php
+------+----------------------------------------------------------+| ID | Clock Source |+------+----------------------------------------------------------+| GOES | Geosynchronous Orbit Environment Satellite || GPS | Global Position System || GAL | Galileo Positioning System || PPS | Generic pulse-per-second || IRIG | Inter-Range Instrumentation Group || WWVB | LF Radio WWVB Ft. Collins, CO 60 kHz || DCF | LF Radio DCF77 Mainflingen, DE 77.5 kHz || HBG | LF Radio HBG Prangins, HB 75 kHz || MSF | LF Radio MSF Anthorn, UK 60 kHz || JJY | LF Radio JJY Fukushima, JP 40 kHz, Saga, JP 60 kHz || LORC | MF Radio LORAN C station, 100 kHz || TDF | MF Radio Allouis, FR 162 kHz || CHU | HF Radio CHU Ottawa, Ontario || WWV | HF Radio WWV Ft. Collins, CO || WWVH | HF Radio WWVH Kauai, HI || NIST | NIST telephone modem || ACTS | NIST telephone modem || USNO | USNO telephone modem || PTB | European telephone modem |+------+----------------------------------------------------------+
Timezones
NewZealand'sChathamIslands(UTC+12:45)
Nepal(UTC+05:45)
Indiana,USA
• twoFmezones
• daylightsavingsince2006
• opposedbyruralcounFes
Australia
# Switzerland. . .# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/SRule Swiss 1941 1942 - MayMon>=1 1:00 1:00 SRule Swiss 1941 1942 - OctMon>=1 2:00 0 -# Zone NAME GMTOFF RULES FORMAT [UNTIL]Zone Europe/Zurich 0:34:08 - LMT1853 Jul 16 # See above comment.
0:29:46 - BMT1894 Jun # Bern Mean Time1:00 Swiss CE%sT 19811:00 EU CE%sT
/tzdata2015g/europetonsofhistoricaldataandcomments!
The tz database attempts to record the history and predicted future ofall computer-based clocks that track civil time.
The Time Zone Database (often called tz or zoneinfo) contains code anddata that represent the history of local time for many representativelocations around the globe. It is updated periodically to reflectchanges made by political bodies to time zone boundaries, UTC offsets,and daylight-saving rules.
IANATimeZoneDatabase
Recap
• HowFmeismeasured(UT,SI)
• HowFmeisusedinPOSIXsystems
• HowFmeisdistributed(shortwaves,NTP,GPS)
• Ididn’tspeakaboutrelaFvity,exoFccalendarsorFmeonotherplanets…
ReferencesDavidW.Allan,NeilAshby,CliffordC.HodgeTheScienceofTimekeeping HewlegPackardApplicaFonNote1289,1997hgp://www.allansFme.com/PublicaFons/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
JamesJespersen,JaneFitz-Randolph FromSundialstoAtomicClocks:UnderstandingTimeandFrequencyNIST,1999hgp://~.boulder.nist.gov/general/pdf/1796.pdf
SteveAllen TheFutureofLeapSecondsinUTC hgp://www.ucolick.org/~sla/leapsecs/